Ab initio valence-electron-only molecular electronic structure calculations: Theory and test applications

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Abstract
A new method for performing molecular electronic structure calculations using electronic wave functions which make explicit reference only to valence electrons is presented. The method contains no adjustable parameters and makes use of projection operators to project out the core function space from the valence‐electron wave function. The energy calculated using this valence‐electron‐only method is guaranteed to converge, with increasing size of orbital basis, upon the valence energy which would be obtained from an equivalent all‐electron calculation using a frozen SCF description of the core electrons. The method may be used in conjunction with any desired treatment of the valence‐electron wave functions (i.e., SCF, CI, MC‐SCF, VB, or VB‐SCF). In this paper, test calculations are performed using an SCF treatment of the valence‐electron wave function for the molecules NaCl, Na2, and Cl2. All‐electron and valence‐electron‐only calculations are compared using differing (i.e., reduced) sizes of orbital basis sets for the valence‐electron‐only calculations. It is shown that valence‐electron‐only calculations can lead to considerable savings in computer time while still providing accurate potential energy curves and valence energies.

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