ANALYTICAL ENERGY GRADIENTS IN 2ND-ORDER MOLLER-PLESSET PERTURBATION-THEORY FOR EXTENDED SYSTEMS

The spin-restricted formulas for the analytical gradients of the secon d-order IC Moller-Plesset perturbation (MP2) energy are presented with in the framework of ab initio crystal orbital theory of infinite one-d imensional lattices (polymers). The coupled perturbed Hartree-Fock equ ation for polymers is solved iteratively using the atomic-orbital-base d algorithms. The MP2 energy and its gradient contributions are evalua ted by the disk-based algorithms with the aid of the two-particle dens ity matrix. The analytical-gradient method at the MP2 level, as well a s the analytical first- and second-derivative methods at the Hartree-F ock (HF) level, is applied to calculate the equilibrium structures and harmonic vibrational frequencies of all-trans polyacetylene. The devi ations of the calculated frequencies from the observed ones for the in -phase C=C stretching modes are reduced by about 70% on going from HF/ 6-31G to MP2/6-31G theory. (C) 1998 American Institute of Physics.