ANALYTICAL ENERGY GRADIENT FOR THE REFERENCE INTERACTION SITE MODEL MULTICONFIGURATIONAL SELF-CONSISTENT-FIELD METHOD - APPLICATION TO 1,2-DIFLUOROETHYLENE IN AQUEOUS-SOLUTION

We have reformulated the reference interaction site model. self-consis tent-held (RISM-SCF) method to calculate the molecular properties of s olute in solutions. The key feature is to introduce the Helmholtz free energy given as a sum of the solute electronic energy estimated by ab initio molecular orbital (MO) methods and the hypernetted-chain (HNC) excess chemical potential of solvation for the extended reference int eraction site model (XRISM) method. The variational conditions for the multiconfigurational self-consistent-held (MCSCF) wave functions were derived. It was also shown that, in the case of the Hartree-Fock (HF) method, the present expression gives the same Fock matrix defined pre viously. Moreover the expression for first derivatives of the free ene rgy with respect to the solute nuclear coordinates were obtained. Samp le calculations for the chemical equilibrium of cis- and trans-isomers of 1,2-difluoroethylene are presented. With the use of analytical ene rgy gradients for the MCSCF and HF methods, the solute molecular geome tries were determined in an aqueous solution. (C) 1996 American Instit ute of Physics.