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Normal mode

A normal mode of a dynamical system is a pattern of motion in which all parts of the system oscillate sinusoidally with the same frequency and fixed phase relations among the components.^[1]^[2] These modes emerge in linear systems with multiple degrees of freedom, such as coupled oscillators, where the equations of motion can be decoupled into independent harmonic oscillations through eigenvalue analysis of the system's dynamical matrix.^[1] The frequencies of normal modes, known as eigenfrequencies, are determined by the system's physical parameters, like masses and spring constants, and the general solution for arbitrary initial conditions is a linear superposition of these modes.^[1]^[2] Normal modes are exemplified in simple coupled systems, such as two pendulums linked by a spring, which exhibit two distinct modes: an in-phase mode where both pendulums oscillate together at the natural frequency \omega_0 = \sqrt{g/\ell} of an isolated pendulum, with the spring remaining unstretched, and an out-of-phase mode where they oscillate oppositely at a higher frequency \omega = \sqrt{\omega_0^2 + 2\kappa/m}, where \kappa is the spring constant and m the mass.^[1]^[2] Similar patterns appear in strings or membranes, where normal modes form standing waves with nodes and antinodes, as seen in the fundamental mode (one loop) and higher harmonics of a vibrating string fixed at both ends. Beyond these mechanical examples, normal modes play a central role across physics. In molecular physics and chemistry, they describe collective vibrational motions of atoms in polyatomic molecules, where each mode involves all atoms moving with the same frequency but varying amplitudes, enabling the analysis of infrared spectra and bond strengths.^[3]^[4] In solid-state physics, lattice vibrations are quantized as phonons, which are normal modes that govern heat capacity, thermal conductivity, and electron-phonon interactions in crystals.^[5] In acoustics and wave propagation, normal modes determine resonant frequencies in enclosures like organ pipes or rooms, influencing sound fields and standing wave patterns in bounded media.^[6] These applications highlight normal modes' utility in simplifying complex oscillatory phenomena into tractable, independent components.^[1]

Core Concepts

Definition and Basic Principles

In classical mechanics, a normal mode refers to a specific pattern of oscillatory motion in a multi-degree-of-freedom system where all components oscillate sinusoidally at the same frequency and maintain fixed phase relationships and displacement ratios among themselves. This independent motion allows the complex coupled dynamics of the system to be decomposed into a superposition of simpler, uncoupled harmonic oscillators, each corresponding to a distinct normal mode. Such modes are particularly relevant in linear systems undergoing small oscillations around a stable equilibrium, where nonlinear effects can be neglected.^[1] The basic principles of normal modes emerge from the mathematical structure of the system's equations of motion, derived typically from Lagrangian mechanics for conservative systems. Consider a system with generalized coordinates \mathbf{q}, kinetic energy T = \frac{1}{2} \dot{\mathbf{q}}^T \mathbf{M} \dot{\mathbf{q}} (where \mathbf{M} is the symmetric positive-definite mass matrix), and potential energy V \approx \frac{1}{2} \mathbf{q}^T \mathbf{K} \mathbf{q} for small displacements (where \mathbf{K} is the symmetric stiffness matrix from the quadratic expansion of V around equilibrium). The Euler-Lagrange equations yield the second-order matrix differential equation \mathbf{M} \ddot{\mathbf{q}} + \mathbf{K} \mathbf{q} = 0. Assuming time-harmonic solutions of the form \mathbf{q}(t) = \mathbf{a} \cos(\omega t + \phi), where \mathbf{a} is the mode shape vector and \omega is the angular frequency, substitution gives (- \omega^2 \mathbf{M} + \mathbf{K}) \mathbf{a} = 0, or equivalently, \mathbf{K} \mathbf{a} = \omega^2 \mathbf{M} \mathbf{a}. This is a generalized eigenvalue problem, with eigenvalues \omega^2 representing the squared normal frequencies and eigenvectors \mathbf{a} the corresponding mode shapes.^[1] The normal frequencies are determined by solving the characteristic equation \det(\mathbf{K} - \omega^2 \mathbf{M}) = 0, a polynomial of degree equal to the number of degrees of freedom n in \omega^2. For an n-degree system, there are n real, non-negative eigenvalues \omega_k^2 (assuming stability, i.e., \mathbf{K} positive semi-definite), yielding n orthogonal normal modes that form a complete basis for the motion. The orthogonality follows from the symmetry of \mathbf{M} and \mathbf{K}, allowing the general solution \mathbf{q}(t) = \sum_k c_k \mathbf{a}_k \cos(\omega_k t + \phi_k) to be expressed as a linear combination of these modes, decoupling the dynamics into independent oscillators. This framework simplifies analysis by transforming to normal coordinates aligned with the eigenvectors.^[1] The theoretical foundation of normal modes traces its origins to 18th- and 19th-century developments in analytical mechanics, particularly Joseph-Louis Lagrange's treatment of small oscillations around equilibrium in Mécanique Analytique (1788), where he formulated the equations for coupled systems,^[7] and Lord Rayleigh's extension to vibrational problems in The Theory of Sound (1877–1878), which emphasized harmonic decompositions in continuous media.^[8]

Mode Numbers and Nodes

Normal modes are typically labeled using integer indices, denoted as n = 1, 2, [3, \dots](/page/3_Dots), where n = 1 corresponds to the fundamental mode with the lowest natural frequency, and higher indices represent modes of increasing frequency.^[1] In systems exhibiting symmetry, such as circular or spherical geometries, degeneracy can occur, where multiple distinct modes share the same frequency due to rotational invariance.^[9] Nodes represent locations within the system where the displacement amplitude remains zero for a particular normal mode, effectively dividing the system into regions of oscillatory motion. In one-dimensional systems, such as a vibrating string fixed at both ends, the n-th mode features n-1 nodes between the fixed endpoints, with the number of antinodes equal to n.^[10] In two- or three-dimensional systems, like vibrating membranes or plates, these zero-displacement loci extend to nodal lines or surfaces, which form patterns that constrain the mode's vibrational structure.^[11] The mode shapes, represented as vectors \boldsymbol{\phi}_n, exhibit orthogonality, meaning \boldsymbol{\phi}_m \cdot \boldsymbol{\phi}_n = 0 for m \neq n, which ensures that the motions of different modes are decoupled and do not exchange energy. These mode shapes are often normalized such that \boldsymbol{\phi}_n \cdot \boldsymbol{\phi}_n = 1 (or, in mass-weighted formulations, \boldsymbol{\phi}_m^T M \boldsymbol{\phi}_n = \delta_{mn}, where M is the mass matrix and \delta_{mn} is the Kronecker delta), facilitating the decomposition of general vibrations into independent modal contributions.^[1]

Applications in Classical Mechanics

Coupled Oscillators

In coupled oscillator systems, multiple degrees of freedom interact through connecting elements, such as springs, leading to collective motions that can be decoupled into independent normal modes. A classic setup involves two identical masses m attached to fixed walls by springs of constant k and connected to each other by a spring of constant \kappa. The equations of motion for displacements x_1 and x_2 are m \ddot{x}_1 = - (k + \kappa) x_1 + \kappa x_2 and m \ddot{x}_2 = - (k + \kappa) x_2 + \kappa x_1. Assuming harmonic solutions x_i = A_i e^{i \omega t}, the system yields two normal modes: a symmetric mode where x_1 = x_2 with frequency \omega_s = \sqrt{k/m}, and an antisymmetric mode where x_1 = -x_2 with frequency \omega_a = \sqrt{(k + 2\kappa)/m}.^[12] A specific example is two identical pendulums of length \ell and mass m, coupled by a spring of constant \kappa connecting the bobs. For small oscillations, the equations of motion are m \ddot{x}_1 = - (mg/\ell) x_1 - \kappa (x_1 - x_2) and m \ddot{x}_2 = - (mg/\ell) x_2 - \kappa (x_2 - x_1), where x_1, x_2 are horizontal displacements. The symmetric normal mode has both pendulums oscillating in phase with frequency \omega_\text{sym} = \sqrt{g/\ell}, as the coupling spring remains unstretched. The antisymmetric mode has them oscillating out of phase with frequency \omega_\text{asym} = \sqrt{g/\ell + 2\kappa/m}, where the spring stretches and compresses maximally. These frequencies arise from solving the eigenvalue problem of the coupled system, decoupling the motion into independent oscillators.^[1] Superposition of these normal modes with nearby frequencies produces beats, where energy oscillates between the oscillators at the beat frequency \nu_\text{beat} = |\nu_1 - \nu_2|/2. For weak coupling (\kappa \ll k), \omega_s \approx \omega_a \approx \sqrt{k/m}, and the motion appears as periodic energy transfer, observable as one mass nearly stopping while the other oscillates fully. This phenomenon illustrates how initial conditions excite multiple modes, leading to quasi-periodic behavior that averages to the uncoupled frequency over time.^[12] For N coupled oscillators, the system generalizes to a set of N coupled differential equations, expressible in matrix form as M \ddot{\mathbf{X}} = -K \mathbf{X}, where M is the mass matrix and K is the stiffness matrix. Normal modes are the eigenvectors of M^{-1} K, with corresponding eigenvalues yielding N distinct frequencies \omega_n. These modes form an orthogonal basis, allowing any initial condition \mathbf{X}(0), \dot{\mathbf{X}}(0) to be expanded as a linear combination. The general solution for the coordinates is

x_i(t) = \sum_{n=1}^N a_n \phi_{n,i} \cos(\omega_n t + \delta_n),

where \phi_{n,i} is the i-th component of the n-th mode eigenvector, and amplitudes a_n, phases \delta_n are determined from initial conditions via projection onto the modes. This decomposition decouples the dynamics, simplifying analysis of complex interactions.^[1]

Standing Waves in Discrete and Continuous Systems

Standing waves in discrete systems, such as a one-dimensional chain of masses connected by springs, represent normal modes where the entire system oscillates at a single frequency. In this model, each mass m is linked by springs with stiffness k, and the dispersion relation governing wave propagation is given by \omega(k) = 2 \sqrt{k/m} \left| \sin(ka/2) \right|, where k is the wave number and a is the lattice spacing. Standing modes arise when boundary conditions, such as fixed ends, are imposed, quantizing the allowed wave numbers and leading to discrete frequencies that form the normal modes of the system. This discrete setup approximates the behavior of coupled oscillators but extends to wave-like phenomena as the number of masses increases. For finite chains with fixed boundaries, the normal modes are characterized by sinusoidal displacements with nodes at the ends, and the frequencies follow from solving the eigenvalue problem of the coupled equations of motion. In the limit of small lattice spacing, this discrete model transitions to a continuous medium, where the dispersion relation approaches the linear form \omega = c k typical of long-wavelength waves. In continuous systems, such as a vibrating string under tension, normal modes manifest as standing waves satisfying the one-dimensional wave equation \frac{\partial^2 u}{\partial t^2} = c^2 \frac{\partial^2 u}{\partial x^2}, where c = \sqrt{T/\mu} is the wave speed, T is the tension, and \mu is the linear density. The general solution for a string of length L with fixed ends yields normal modes of the form u_n(x,t) = \sin\left(\frac{n\pi x}{L}\right) \cos(\omega_n t), with frequencies \omega_n = \frac{n\pi c}{L} for integer n. These modes are orthogonal and can be superposed to describe arbitrary initial conditions, forming a complete basis for the system's vibrations. Boundary conditions profoundly influence the mode shapes and frequencies in continuous systems. For fixed ends, as in a guitar string, the displacement vanishes at x=0 and x=L, enforcing nodal points there and producing the sinusoidal modes described above. In contrast, free ends, such as those on a free-floating rod, allow non-zero displacement and slope at the boundaries, resulting in cosine-like mode shapes u_n(x,t) = \cos\left(\frac{n\pi x}{L}\right) \cos(\omega_n t) with \omega_n = \frac{n\pi c}{L} for n=0,1,2,\dots, where the n=0 mode corresponds to rigid-body translation. These differences highlight how boundaries dictate the quantization of modes, with fixed conditions typically yielding higher fundamental frequencies than free ones for the same length and material properties. The role of these standing normal modes is exemplified in musical instruments, where the harmonics of a string under fixed boundaries produce the pitched tones of instruments like violins or pianos; the fundamental mode (n=1) gives the lowest pitch, while overtones (n>1) enrich the timbre. This discrete-to-continuous transition underscores the universality of normal modes, bridging simple lattice vibrations to macroscopic wave phenomena in extended media.

Vibrations in Elastic Solids

In elastic solids, vibrations arise from the dynamic response of the material to deformations, governed by the equations of elastodynamics under the assumption of small displacements and linear material behavior. For homogeneous and isotropic solids obeying Hooke's law, the fundamental relation connects stress \sigma and strain \epsilon via \sigma_{ij} = \lambda \delta_{ij} \epsilon_{kk} + 2\mu \epsilon_{ij}, where \lambda and \mu are the Lamé constants, \delta_{ij} is the Kronecker delta, and \epsilon_{ij} = \frac{1}{2} (\partial_i u_j + \partial_j u_i) is the strain tensor derived from the displacement field \mathbf{u}. The motion is then described by the balance of linear momentum, expressed as Navier's equations: \nabla \cdot \sigma = \rho \frac{\partial^2 \mathbf{u}}{\partial t^2}, or in expanded form for body forces \mathbf{f}, (\lambda + \mu) \nabla (\nabla \cdot \mathbf{u}) + \mu \nabla^2 \mathbf{u} + \mathbf{f} = \rho \frac{\partial^2 \mathbf{u}}{\partial t^2}, where \rho is the mass density. These equations couple compressional and shear deformations, enabling complex wave propagation in three-dimensional continua such as plates, shells, and bulk solids.^[13] Normal modes in elastic solids manifest as distinct types depending on the geometry and boundary conditions, with rods exemplifying one-dimensional approximations to continuum behavior. In slender rods, longitudinal modes involve axial compression and extension, propagating at speed \sqrt{(\lambda + 2\mu)/\rho}; transverse modes correspond to shear or flexural bending at speed \sqrt{\mu/\rho}; and torsional modes feature twisting deformations, also at speed \sqrt{\mu/\rho}. For a free-free rod of length L, the frequencies of longitudinal and torsional modes are f_n = n c / (2L), where c is the respective wave speed and n is a positive integer labeling the mode order. In thin plates, Kirchhoff-Love theory simplifies the three-dimensional problem by assuming negligible transverse shear, leading to transverse vibrational modes governed by the biharmonic equation D \nabla^4 w + \rho h \frac{\partial^2 w}{\partial t^2} = 0, where w is the transverse deflection, D = Eh^3 / [12(1 - \nu^2)] is the flexural rigidity (E Young's modulus, h thickness, \nu Poisson's ratio), and the operator \nabla^4 captures bending stiffness. These modes decouple from in-plane vibrations for thin geometries, focusing on flexural waves.^[14]^[15] A representative example is the free vibration of a uniform Euler-Bernoulli beam, where normal modes are computed to predict resonant frequencies critical for structural design. The Rayleigh-Ritz method approximates the mode shapes as a series of admissible functions, such as polynomials or static beam functions, minimizing the Rayleigh quotient \omega^2 = \frac{\int EI (\phi'')^2 dx}{\int \rho A \phi^2 dx} to yield upper-bound eigenvalues, with continuity enforced via Lagrange multipliers for composite beams. For a simply supported uniform beam, this approach converges to the first-mode frequency of approximately 9.87 rad/s (normalized) with fewer than 10 terms per segment, closely matching exact solutions derived from the characteristic equation involving trigonometric and hyperbolic functions. In cases with circular cross-sections, exact solutions for higher-order modes incorporate Bessel functions to satisfy radial boundary conditions, though Rayleigh-Ritz remains versatile for irregular geometries.^[16] The identification of normal modes reduces the time-dependent problem to a spatial eigenvalue equation by assuming harmonic time dependence in the displacement: \mathbf{u}(\mathbf{r}, t) = \boldsymbol{\phi}(\mathbf{r}) e^{i \omega t}, where \boldsymbol{\phi} is the mode shape and \omega the angular frequency. Substituting into Navier's equations for free vibrations (\mathbf{f} = 0) yields the generalized eigenvalue problem -\rho \omega^2 \boldsymbol{\phi} = \mu \nabla^2 \boldsymbol{\phi} + (\lambda + \mu) \nabla (\nabla \cdot \boldsymbol{\phi}), subject to boundary conditions (e.g., traction-free surfaces \boldsymbol{\sigma} \cdot \mathbf{n} = 0) that discretize the continuous spectrum into countable eigenfrequencies \omega_n and eigenfunctions \boldsymbol{\phi}_n. This formulation ensures orthogonality of modes, facilitating superposition for general responses.^[17] In engineering applications, modal analysis extends these concepts to damped systems using finite element methods (FEM), which discretize the solid into elements and solve the assembled eigenvalue problem [K - \omega^2 M] \boldsymbol{\phi} = 0, where K and M are stiffness and mass matrices. Damping is incorporated proportionally (Rayleigh damping) or via viscous terms in the equations of motion, yielding complex eigenvalues whose imaginary parts represent decay rates, essential for predicting resonance in structures like aircraft components. FEM enables computation of mode participation factors and damping ratios, with modal superposition reducing computational cost for transient simulations.^[18]^[19]

Applications in Quantum Mechanics

Normal Modes in Quantum Harmonic Systems

In quantum harmonic systems, particularly for multi-particle configurations such as molecules, normal modes are quantized by promoting the classical normal coordinates to quantum operators. The transformation from Cartesian coordinates x_j to normal coordinates is given by Q_k = \sum_j U_{jk} x_j, where the matrix U is obtained by diagonalizing the Hessian matrix of second derivatives of the potential energy at the equilibrium geometry, ensuring the modes are decoupled harmonic oscillators.^[20] In the quantum treatment, these Q_k become operators \hat{Q}_k, and the vibrational Hamiltonian separates into independent terms for each mode. For a non-linear molecule with N atoms, there are $3N-6 vibrational degrees of freedom, each corresponding to a quantum harmonic oscillator.^[20] The quantized Hamiltonian for the system takes the form \hat{H} = \sum_n \hbar \omega_n \left( \hat{a}_n^\dagger \hat{a}_n + \frac{1}{2} \right), where \hat{a}_n and \hat{a}_n^\dagger are the annihilation and creation operators for the n-th normal mode with frequency \omega_n, derived via second quantization of the harmonic potential.^[21] This formulation arises from the classical precursor of coupled oscillators, where the normal modes diagonalize the equations of motion, now elevated to operator algebra satisfying [\hat{a}_n, \hat{a}_m^\dagger] = \delta_{nm}. The energy levels for each mode are E_v = \hbar \omega_n \left( v + \frac{1}{2} \right), with quantum number v = 0, 1, 2, \dots, leading to a total zero-point energy of \frac{1}{2} \sum_k \hbar \omega_k even at absolute zero, which contributes to molecular stability and spectroscopic observables.^[20] In spectroscopy, the harmonic approximation imposes selection rules, such as \Delta v = \pm 1 for fundamental transitions in infrared or Raman spectra when the mode alters the dipole moment or polarizability, respectively, enabling precise assignment of vibrational frequencies.^[20] Deviations from harmonicity, such as bond stretching limits or interactions between modes, are treated as perturbations to the harmonic Hamiltonian, introducing corrections like overtones (\Delta v > 1) and combination bands, which broaden spectral lines and shift frequencies.^[22] This perturbative approach, often using vibrational perturbation theory, refines predictions for anharmonic effects while retaining the independent-oscillator framework for the unperturbed system.^[22]

Quantum Field Theory and Normal Modes

In quantum field theory, normal modes play a central role in the quantization of relativistic fields, where the classical field solutions are expanded in terms of these modes, and the resulting coefficients are promoted to creation and annihilation operators that describe particle excitations.^[23] For a free scalar field obeying the Klein-Gordon equation, the field operator is expressed as a superposition of normal modes:

\phi(x,t) = \int \frac{d^3k}{(2\pi)^3} \frac{1}{\sqrt{2\omega_k}} \left[ a_{\mathbf{k}} u_{\mathbf{k}}(x) e^{-i\omega_k t} + a^\dagger_{\mathbf{k}} u^*_{\mathbf{k}}(x) e^{i\omega_k t} \right],

where \omega_k = \sqrt{|\mathbf{k}|^2 + m^2}, u_{\mathbf{k}}(x) = e^{i\mathbf{k}\cdot\mathbf{x}} are plane-wave normal modes, and the Hermitian conjugate (h.c.) term ensures reality.^[23] This expansion diagonalizes the Hamiltonian into independent harmonic oscillators for each mode, with the canonical commutation relations [\phi(\mathbf{x},t), \dot{\phi}(\mathbf{y},t)] = i\delta^3(\mathbf{x}-\mathbf{y}) imposing [a_{\mathbf{k}}, a^\dagger_{\mathbf{k}'}] = (2\pi)^3 \delta^3(\mathbf{k}-\mathbf{k}') on the operators.^[23] Similar mode expansions apply to fermionic fields like the Dirac field, where anticommutators replace commutators to enforce Fermi statistics.^[23] These quantized normal modes form the basis of Fock space, the Hilbert space of multi-particle states built by applying creation operators a^\dagger_{\mathbf{k}} to the vacuum |0\rangle, satisfying a_{\mathbf{k}}|0\rangle = 0 for all \mathbf{k}, thus representing particles as excitations of the underlying field.^[23] In quantum electrodynamics (QED), the photon field is quantized via transverse electromagnetic normal modes, expanding the vector potential in Fourier modes with two polarization states, leading to massless spin-1 particles with commutation relations analogous to the scalar case but projected onto transverse directions to satisfy Gauss's law.^[24] Likewise, in condensed matter physics, phonons emerge as quantized normal modes of lattice vibrations, where the displacement field of atoms is expanded in sound-wave-like modes, yielding bosonic quasiparticles with linear dispersion in the long-wavelength limit. The vacuum state in this framework is not empty but features fluctuations due to the nonzero zero-point energy of all modes, \langle 0 | H | 0 \rangle = \frac{1}{2} \sum_{\mathbf{k}} \omega_k, which must be regularized.^[23] A striking observable consequence is the Casimir effect, where the sum over allowed modes between two conducting plates yields an attractive force arising from the difference in vacuum energy compared to free space, experimentally verified and scaling as F \propto -\frac{\hbar c \pi^2 A}{240 d^4} for plate area A and separation d. This mode-sum regularization highlights how normal modes underpin relativistic particle physics and quantum vacuum phenomena.^[25]

Applications in Other Fields

Seismology and Earth Sciences

In seismology, normal modes describe the free oscillations of the Earth following large earthquakes, representing standing waves that propagate globally within the planet's elastic structure. These oscillations arise from the sudden release of strain energy, exciting the Earth's natural resonant frequencies, and can persist for days or weeks depending on attenuation. The Earth, modeled as an elastic body with spherical symmetry, supports two primary types of normal modes: spheroidal modes, which involve radial and tangential displacements akin to compressional (P) and shear (S) wave motions, and toroidal modes, which are purely tangential and divergence-free, analogous to shear waves without volume change. Frequencies of these modes are determined by solving the equations of linear elasticity in spherical coordinates, accounting for density, shear modulus, and compressional wave speeds varying with depth.^[26] Spheroidal modes are denoted as _n S_l, where n is the overtone number (starting from 0 for fundamental modes), and l is the angular degree labeling the number of nodal surfaces; toroidal modes are _n T_l. The fundamental spheroidal mode _0 S_2 has a period of approximately 54 minutes, while overtones extend to higher n and degrees up to l > 20, with periods ranging from minutes to hours. These modes were first convincingly observed in recordings of the 1960 great Chilean earthquake (magnitude 9.5) by Benioff, Press, and Smith, who identified multiple resonant peaks in long-period strain and displacement seismograms, confirming the Earth's global vibrational spectrum.^[27] The eigenfrequencies \omega for full three-dimensional normal modes satisfy the vector equation

\nabla \times (\nabla \times \mathbf{u}) + \frac{\omega^2}{c^2} \mathbf{u} = 0,

where \mathbf{u} is the displacement vector and c is the local shear wave speed (with modifications for P-SV coupling in spheroidal modes); this derives from the elastodynamic wave equation in the frequency domain, subject to free-surface and continuity boundary conditions. In the long-wavelength limit, solutions reduce to Love's equations for Love (toroidal) and Rayleigh (spheroidal) surface waves as special cases. To model observed seismograms, normal mode theory computes synthetic traces by summing modal contributions, typically using the Preliminary Reference Earth Model (PREM) for radial structure, which incorporates anelastic attenuation to match decaying amplitudes.90046-7)^[28]

Molecular Spectroscopy and Vibrational Modes

In molecular spectroscopy, normal modes describe the independent vibrational motions of atoms within a polyatomic molecule, providing insight into its structure and bonding. For a nonlinear molecule with N atoms, there are $3N - 6 vibrational degrees of freedom, corresponding to the total $3N Cartesian coordinates minus 3 translational and 3 rotational modes.^[29] These modes are classified by symmetry and type, such as stretching (changes in bond lengths), bending (changes in bond angles), scissoring, rocking, wagging, and twisting, which facilitate the interpretation of vibrational spectra.^[29] A key aspect of normal modes in spectroscopy is their activity in infrared (IR) and Raman scattering, determined by changes in the molecule's dipole moment or polarizability, respectively. For example, in the linear triatomic CO₂ molecule, which has 4 vibrational modes ($3 \times 3 - 5 = 4), the symmetric stretch is IR inactive due to no net dipole change but Raman active, while the asymmetric stretch is IR active at approximately 2349 cm⁻¹ and Raman inactive. The two degenerate bending modes are both IR and Raman active around 667 cm⁻¹. These distinctions arise from the molecule's D_{\infty h} symmetry, enabling structural analysis through observed spectral bands.^[30] To analyze normal modes, computational techniques explore the potential energy surface (PES), a multidimensional hypersurface representing the molecule's total energy as a function of atomic coordinates, where minima correspond to equilibrium geometries and curvatures define vibrational frequencies.^[31] The Wilson GF method, introduced in the seminal work on molecular vibrations, calculates these modes by transforming internal coordinates into normal coordinates via the kinetic energy matrix G and force constant matrix F, solving for eigenvalues that yield frequencies.^[32] Vibrational frequencies are derived from the eigenvalues of the mass-weighted force constant matrix. In mass-weighted Cartesian coordinates, the Hessian matrix H (second derivatives of the potential energy) is diagonalized, with eigenvalues \lambda_k related to the harmonic frequencies by

\nu_k = \frac{1}{2\pi} \sqrt{\lambda_k},

where \nu_k is in Hz; wavenumbers in cm⁻¹ are obtained by scaling \nu_k / c with c as the speed of light.^[33] This quantum harmonic approximation underpins spectral assignments, linking observed transitions to molecular structure.^[34]

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Molecular Vibrations: The Theory of Infrared and Raman Vibrational Spectra Dover Books on Chemistry. Authors, E. Bright Wilson, J. C. Decius, Paul C. Cross.
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3.2: Normal Modes of Vibration - Chemistry LibreTexts
Apr 8, 2021 · Hence, to perform a normal-mode analysis of a molecule, one forms the mass-weighted Hessian matrix and then finds the $3N-5$ or $3N-6$ non- ...Missing: quantization | Show results with:quantization
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Vibrational Analysis in Gaussian
Oct 29, 1999 · The purpose of this document is to describe how Gaussian calculates the reduced mass, frequencies, force constants, and normal coordinates