A COMBINED DENSITY-FUNCTIONAL AND INTRINSIC REACTION COORDINATE STUDY ON THE GROUND-STATE ENERGY SURFACE OF H2CO

Approximate density functional theory (DFT) has been used to study four elementary processes on the singlet ground state energy surface of H2CO. The elementary steps include the elimination of H-2 and CO from formaldehyde, the transformation of formaldehyde to transhydroxymethylene, the isomerization of trans-hydroxymethylene to cis-hydroxymethylene, and the 1,2 elimination of H2 from cis-hydroxymethylene. The DFT studies were based on the local density approximation (LDA) as well as a nonlocal self-consistent field (NL-SCF) extension in which Perdew's correlation correction and Becke's exchange correction were added to LDA. Fully optimized structures as well as harmonic vibrational frequencies have been evaluated for all of the stationary points corresponding to the reactants, products, and transition states for the four reactions within the LDA and NL-SCF approximations. The four reactions have in addition been studied by the intrinsic reaction coordinate (IRC) method in which stationary points on the potential energy surface are connected by a steepest energy path. It is concluded that the NL-SCF-method affords as good an overall fit to experiment as the fourth-order Moller-Plesset single, double, triple, and quadruple excitations [MP4(SDTQ)]/6-31G**//MP2/6-31G* scheme for the S0 surface of H2CO. The DFT methods tend to afford energies for the stationary points which are too low compared to the separate species H-2 + CO, whereas the corresponding MP4 energies are too high. This is in particular the case for the local LDA method. We attribute this trend to a bias in favor of bond formation among the DFT methods. The reaction paths for LDA and NL-SCF were found to be very similar, and it is suggested that one might combine geometries evaluated by the less demanding LDA method with energies obtained by the more accurate and involved NL-SCF scheme.