Molecular Vibrations C. David Sherrill School of Chemistry and Biochemistry Georgia Institute of Technology

Transcription

1 Molecular Vibrations C. David Sherrill School of Chemistry and Biochemistry Georgia Institute of Technology

2 Why Estimate Molecular Vibrations? Simulation of vibrational spectrum (identification of molecules) Vibrational corrections to enthalpy (Small) vibrational corrections to polarizability and other properties Understanding of vibrational motion could assist dynamics experiments and mode-selective chemistry

3 Small Vibrations in Classical Mechanics The classic reference is Wilson, Decius, and Cross, Molecular Vibrations (Dover, New York, 1980). Cheap book, makes a good reference. Let us focus on purely classical systems at first; all the results carry over to quantum mechanics. For small vibrations, the motion of atom α away from its equilibrium value may be described by x α, y α, z α, with kinetic energy T = 1 2 N M α α=1 ( d xα dt ) 2 + ( d yα dt ) 2 + ( d zα dt ) 2

4 If we switch to mass-weighted coordinates, such as q 1 = M1 x 1, q 2 = M 1 y 1, q 3 = M 1 z 1, q 4 = M 2 x 2, etc., then the kinetic energy operator becomes simpler since the mass factors are now absorbed 3N T = 1 2 i=1 V = V 0 + q 2 i 3N i=1 ( V q i ) 0q i N i=1 ( 2 V q i q j ) 0 q i q j + (1) Remember that at equilibrium, ( V/ q i ) 0 = 0; we can also set V 0 = 0. Also abbreviate ( 2 V/ q i q j ) 0 as just f ij.

5 Newton s Equations of Motion We can rewrite Newton s equations of motion as or d dt T q j + V q j = 0 j = 1,2,,3N q j + 3N i=1 f ij q i = 0 A possible solution to this equation is q i = a i cos ( λt+φ ) where the angular frequency is λ; this is just k/m in harmonic oscillator the m has been absorbed by the massweighted coordinate system used here!

6 Substitute the last expression into the differential equations to get 3N i=1 (f ij δ ij λ)a i = 0 j = 1,2,,3N or in matrix notation, just F a = λ a. This is an eigenvalue equation! We have a solution to this system of 3N linear equations only if λ has special values obtainable from the secular determinant f 11 λ f 12 f 13 f 1,3N f 21 f 22 λ f 23 f 2,3N... f 3N,1 f 3N,2 f 3N,3 f 3N,3N λ.. = 0

7 Normal Modes of Vibration The matrix eigenvalue equation is equivalent to matrix diagonalization which is equivalent to solving the secular determinant for each λ (N of them). Once we have the eigenvalues λ k we can get the corresponding eigenvectors a k, giving the motion of each atom for the given eigenvalue λ k : ) q ik = a ik cos ( λ k t+φ k. The eigenvectors a k are the normal modes of vibration. For each normal mode, all the atoms move with the same frequency and phase, but with different amplitudes.

8 Normal Coordinates We can define a new set of coordinates using the normal modes. This gives us the normal coordinates Q k = 3N i=1 a ik q i k = 1,2,,3N Since the eigenvectors of a real, symmetric matrix (F) are orthogonal, T and V become diagonal (no cross terms): 3N T = 1 2 k=1 3N Q 2 k V = 1 2 k=1 λ k Q 2 k The Hamiltonian is separable in this representation!

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10 Polyatomic Molecules What happens for quantum mechanics, and for polyatomic molecules? Use Harmonic Oscillator model. 3N-6 frequencies (3N-5 for linear molecules); the rest are translations and rotations with zero frequency In normal mode coordinates, Hamiltonian is separable: wavefunction is a product and energy is a sum. Total vibrational energy is iω i h(v i +1/2) Minimum energy (due to uncertainty principle) is zero point vibrational energy (ZPVE or ZPE), where v i = 0 for all i. ZPVE = 2 h 1 iω i

11 How Would We Get Harmonic Frequencies for a Molecule? Easy just diagonalize the second derivative matrix F, called the Hessian. The frequencies ω i are the square roots of the eigenvalues, λ i. Where do we get F? Recall f ij = ( 2 V/ q i q j ) Potential energy V is just E e (B.O. approximation!): Need E 2 e/ q i q j. Compute second derivative of E e in terms of of Cartesian displacements (x α, y α, z α, call them q i ) and it s easy to transform to mass-weighted coordinates, using

12 F = M 1/2 F M 1/2. How do we get 2 E e / x α y β, etc? Need second derivative of electronic energy vs nuclear coordinates. Can compute analytically (using formula) or numerically from finite differences of energies or gradients: 2 [( ) ( ) ] E e Ee Ee / x x α y β y β α. xα =x α0 + x α y β xα =x α0 x α

13 Analytic Hessian Better than Numerical Analytic Hessian might cost 10-30x cost of energy; analytic gradient costs maybe 1.5-2x. Can need gradients from many displaced geometries up to 6N (+ and - for each of 3N coordinates) unless reduced by point group symmetry Numerical Hessian contains numerical errors (divide small number by small number) (Can land on wrong solution if displacement drops symmetry)

14 Availability of Analytic Derivatives Method Gradient Hessian HF, DFT Y Y CI Y N CCSD, CCSD(T) Y S MP2 Y S CASSCF Y S CIS Y Y EOM-CCSD S N TD-DFT S N S = available in some packages; Y = widely available

15 Approximate Average Errors in Harmonic Frequencies (Using polarized double and triple zeta basis sets) Method Error HF 11% CISD 4-6% CCSD 1-4% CCSD(T) 1-3% Anharmonicity accounts for another 2-3% difference from experimental fundamental frequencies. Many workers employ scaling factors for each level of theory to better predict fundamental frequencies.

16 100 Spectra of SiC2H2 isomers: DZP CCSD(T) Silapropadienylidene Silacyclopropyne Experiment Intensity Frequency (1/cm)

17 Scaling ZPVE s In an enlightening paper, Grev, Janssen, and Schaefer[J. Chem. Phys. 95, 5128 (1991)] showed that using scaled fundamental frequencies to estimate the ZPVE is not necessarily better than using unscaled frequencies. The reason is anharmonicity. If ZPVE s use scaling, they should have a different scaling factor than the individual frequencies. G(v) = ( ω r v r + 1 ) ( χ rs v r + 1 )( v s + 1 ) +, r r s harm = G(0) ZPVE harm = 1 4 fund = G(0) ZPVE fund = 3 4 r r χ rr χ rr 1 4 r>s r>s χ rs. χ rs.

18 Characterization of Stationary Points A stationary point is a geometry q for which the gradient E e ( q)/ q i for all coordinates q i : can be a (global or local) PES minimum, transition state, or higher order saddle point The Hessian Index is the number of negative force constants (corresponding to imaginary vibrational frequencies, often printed as negative frequencies) For a minimum, verify that there are no imaginary frequencies For a transition state, verify there is exactly one unique imaginary frequency

19 Printed by David Sherrill Dec 15, 00 14:54 h2o.freq.out Page 1/5 *************************************************************** Running Q Chem cicero.chemistry.gatech.edu on Fri Dec 15 14:51:29 EST Version: /usr/local/qchem /bin/qchem /usr/local/qchem /exe/progman.exe *************************************************************** Welcome to Q Chem A Quantum Leap Into The Future Of Chemistry J. Kong, C. A. White, A. I. Krylov, C. D. Sherrill, R. D. Adamson, T. R. Furlani, M. S. Lee, A. M. Lee, S. R. Gwaltney, T. R. Adams, H. Dachsel, W. M. Zhang, P. P. Korambath, C. Ochsenfeld, A. T. B. Gilbert, G. S. Kedziora, D. R. Maurice, N. Nair, Y. Shao, N. A. Besley, P. E. Maslen, J. P. Dombroski, J. Baker, E. F. C. Byrd, T. Van Voorhis, M. Oumi, S. Hirata, C. P. Hsu, N. Ishikawa, J. Florian, A. Warshel, B. G. Johnson, P. M. W. Gill, M. Head Gordon, J. A. Pople, Q Chem, Version 2.0, Q Chem, Inc., Export, PA (2000). Intel x86 Linux Version ====================================================================== User input: ====================================================================== $comment Water $end $molecule 0 1 O H1 O OH H2 O OH H1 HOH OH = HOH = $end $rem JOBTYPE FREQ EXCHANGE HF CORRELATION NONE BASIS STO 3G SCF_FINAL_PRINT 2 Energies plus MOS $end ====================================================================== Processing $rem in the input. # Entering fldman.exe on Fri Dec 15 14:51: # Standard Nuclear Orientation (Angstroms) I Atom X Y Z 1 O H H Molecular Point Group C2v NOp = 4 Largest Abelian Subgroup C2v NOp = 4 Nuclear Repulsion Energy = hartrees h2o.freq.out Dec 15, 00 14:54 Page 2/5 There are 5 alpha and 5 beta electrons Requested basis set is STO 3G There are 4 shells and 7 basis functions A cutoff of 1.0D 10 yielded 10 shell pairs There are 34 function pairs # Entering gesman.exe on Fri Dec 15 14:51: # Smallest overlap matrix eigenvalue = 3.63E 01 Multipole matrices computed through 2nd order Guess from superposition of atomic densities Warning: Energy on first SCF cycle will be non variational # Entering scfman.exe on Fri Dec 15 14:51: # A restricted Hartree Fock SCF calculation will be performed using Pulay DIIS extrapolation SCF converges when DIIS error is below 1.0E 08 Cycle Energy DIIS Error E E E E E E E E E E Convergence criterion met SCF time: CPU 0.08 s wall 0.00 s Final Alpha MO Eigenvalues Final Alpha MO Coefficients Final Alpha density matrix Monday April 23, 2001 h2o.freq.out 1/1

20 Printed by David Sherrill Dec 15, 00 14:54 Page 3/ # Entering anlman.exe on Fri Dec 15 14:51: # Analysis of SCF Wavefunction Orbital Energies (a.u.) and Symmetries Alpha MOs, Restricted Occupied A1 2 A1 1 B1 3 A1 1 B2 Virtual A1 2 B1 Beta MOs, Restricted Occupied A1 2 A1 1 B1 3 A1 1 B2 Virtual A1 2 B1 Mulliken Net Atomic Charges Atom Charge (a.u.) 1 O H H Sum of atomic charges = h2o.freq.out Cartesian Multipole Moments Charge (ESU x 10^10) Dipole Moment (Debye) X Y Z Tot Quadrupole Moments (Debye Ang) XX XY YY XZ YZ ZZ Octapole Moments (Debye Ang^2) XXX XXY XYY YYY XXZ XYZ YYZ XZZ YZZ ZZZ Hexadecapole Moments (Debye Ang^3) XXXX XXXY XXYY XYYY YYYY XXXZ XXYZ XYYZ YYYZ XXZZ XYZZ YYZZ XZZZ YZZZ ZZZZ Dec 15, 00 14:54 Page 4/5 # Entering drvman.exe on Fri Dec 15 14:51: # Calculating MO derivatives via CPHF Roots Converged Calculating analytic Hessian of the SCF energy Polarizability Matrix (a.u.) Direct stationary perturbation theory relativistic correction: rels = relv = rel2e = E_rel = h2o.freq.out Hessian of the SCF Energy Gradient time: CPU 0.59 s wall 1.00 s # Entering vibman.exe on Fri Dec 15 14:51: # ********************************************************************** ** ** ** VIBRATIONAL ANALYSIS ** ** ** ** ** ** VIBRATIONAL FREQUENCIES (CM** 1) AND NORMAL MODES ** ** INFRARED INTENSITIES (KM/MOL) ** ** ** ********************************************************************** Frequency: IR Active: YES YES YES IR Intens: Raman Active: YES YES YES X Y Z X Y Z X Y Z O H H STANDARD THERMODYNAMIC QUANTITIES AT K AND 1.00 ATM This Molecule has 0 Imaginary Frequencies Monday April 23, 2001 h2o.freq.out 1/1