Adiabatic correction to the energy of molecular systems: The CPHF equivalent of the Born-Handy formula

The Born-Handy formula, recently shown by Kutzelnigg to be a rigorous expre ssion for the calculation of the adiabatic correction, has been, on the lev el of a ground state SCF wavefunction, reformulated and linked to the coeff icients of the standard coupled perturbed Hartree-Fock (CPHF) method. The c ontribution of the electron correlation via second-order perturbation theor y is also presented. The solution of the corresponding secular equation of the nuclear motion enables the calculation of the adiabatic correction over the particular normal modes. The method offers the possibility of extendin g high-precision calculations of the adiabatic correction to more complex s ystems. Test calculations have been performed for H-2, HD and D-2 and the r esults are in satisfactory agreement with the exact figures. (C) 1999 Elsev ier Science B.V. All rights reserved.