A frozen‐core approximation using reduced core‐basis functions

A frozen‐core approximation (FCA) is proposed using a new orthogonalization method of the valence basis functions to the core orbitals. The orthogonalization is done through corelike basis functions in contrast to the usual Schmidt process in which the valence basis functions are directly orthogonalized to the core orbitals. The FCA using the new orthogonalization is mathematically rigorous and attains saving of the computation using the reduced corelike basis functions. Numerical calculations on diatomic molecules HCl, N2, P2, and Cl2 reproduced well the results of all‐electron calculations. Discussion on the proposed FCA is given especially in connection with the model‐potential methods of Huzinaga and co‐workers.