Molecular calculations with two-center correlated orbitals

The usual building blocks of molecular orbital theory are one-center atomic orbitals. If we instead use two-center orbitals derived from the exact solution for H+2, a simple calculation of the H2 binding energy yields 4.5 eV, with a Hylleraas correlation factor having no adjustable parameters. Variational minimization with respect to a couple of parameters in our trial function then shifts the binding energy to 4.7 eV, very near the experimental value. The approach is also applicable to heteronuclear molecules; e.g., HeH+, whose ground state energy we calculate to be −2.96 hartree with no adjustable parameters. This is to be compared with −2.98 hartree previously calculated using a wavefunction with 83 terms.