AN AB-INITIO CALCULATION OF A 6-DIMENSIONAL POTENTIAL SURFACE FOR THE HCL DIMER

In an earlier paper (P. C. Gomez, P. R. Bunker, A. Karpfen, and H. Lischka, J. Mol. Spectrosc 166, 441-448, 1994) we calculated three-dimensional ab initio potential energy surfaces for the HCl dimer at five different Values of the dimer stretching coordinate R (the two HCl bond lengths were held fixed) and used those potentials in a calculation of the vibrational energies involving the Four large-amplitude modes. We made an adiabatic separation of the dimer stretching coordinate R in determining the vibrational energies. In the present paper, we calculate the energies without making the adiabatic approximation to separate R, and we also calculate the variation of the potential energy with HCl bond lengths. We use these ab initio results to determine the HCl stretching frequencies as a function of the values of the four large-amplitude coordinates and hence generate an effective four-dimensional large-amplitude potential for the (upsilon(1) = 0, upsilon(2) = 0) HCl stretching state. We calculate the vibrational energies for this surface. (C) 1994 Academic Press, Inc.