>The force constants for stretching and bending internal coordinates can be calculated using the attached Fortran program, which is based on the methods described in J. Chem. Phys. 137, 084114 (2012). The original definitions of the adiabatic force constants and the adiabatic vibrational frequencies were described in Int. J. Quant. Chem. 67, 1 (1998); ibid. 67, 11 (1998).
>It is valid for stationary points, and the geometry should be fully optimized using tight optimization convergence criteria. Otherwise, the force constant matrix in internal coordinates is not uniquely defined (for example, see J. Mol. Spectrosc. 205, 227 2001), and the calculated force constants will be isotope-dependent.
>For bending or dihedral angles, two force constants are given, where Ka(d) is scaled by distants and Ka is not. Both of them were reported in literature.
>Bending angle A-B-C:
>Ka(d) = Ka / [R(A,B) * R(B,C)]
>Dihedral angle A-B-C-D:
>Ka(d) = Ka / [d(A,B-C) * d(D,B-C)]
>= Ka * R(B,C) * R(B,C) / [4 * S(A,B,C) * S(B,C,D)]
>d(A,B-C): distance between the atom A and the bond B-C
>S(A,B,C): area of the triangle ABC
>To run the program, a modified PUNCH file from the Firefly frequency calculation is required. Please see the attached PUNCH file about the details.
[ This message was edited on Wed Oct 3 '12 at 10:27am by the author ]