Similarity-transformed Hamiltonians by means of Gaussian-damped interelectronic distances

A method is presented to improve the description of electron correlation in configuration interaction (CI) calculations. In this method, the standard CI expansion is multiplied by a correlation function phi = exp (F) with [GRAPHICS] With this correlation function, the total wavefunction Psi = phipsi exhibits the right behavior when two electrons coalesce while F vanishes for large interelectronic distances. The correlation function is implemented using the methodology of similarity-transformed Hamiltonians and is applied to two-electron systems. A generalization to many-electron systems is indicated. The new method yields more accurate results than standard CI calculations of the energy and interelectronic distance of the He atom. The H-2 molecule was chosen to study the long-range behavior of the correlation function.