QUANTUM-MECHANICAL GEOMETRY OPTIMIZATION IN SOLUTION USING A FINITE-ELEMENT CONTINUUM ELECTROSTATICS METHOD

We present a new algorithm for performing ab initio solution phase geo metry optimizations. The procedure is based on the self consistent-rea ction-field method developed in our laboratory which combines electron ic structure calculations with a finite element formulation of the con tinuum electrostatics problem. A gradient for the total solution phase free energy is obtained by combining different contributions from the gradient of the classical polarization free energy and the derivative s of the quantum mechanical energy. The method used in obtaining the c lassical gradient is based on exact linear algebra relations and a Gre en function formalism due to Handy and Schaefer. Both the classical an d quantum mechanical gradients are validated by comparison with energy finite differences. The result of applications to a number of small o rganic compounds are discussed. Comparisons between the predicted loca tion and depth of the various solution phase minima of the Ramachandra n map for the alanine dipeptide and those reported by Gould et al. are also presented. (C) 1996 American Institute of Physics.