THE SELF-CONSISTENT NONORTHOGONAL GROUP-FUNCTION APPROACH IN REDUCED BASIS FROZEN-CORE CALCULATIONS

The orthogonal group function approach, as based on the Huzinaga equat ion, is extensively applied in reduced basis frozen-core calculations. Although the theory is developed for orthogonal electronic groups, th e use of reduced basis sets prevents strict orthogonality and the form alism is complemented to take, partially, into account nonorthogonalit y (projection factors, projection energy). In the present article, an alternative to this approach, based on the nonorthogonal formalism, is proposed. An orbital equation is derived from the Adams-Gilbert equat ion and the energy is evaluated according to a recent proposal based o n the power-series expansion of the overlap energy. A comparative over view of the orthogonal and nonorthogonal formalisms is presented and t he results of reduced basis frozen-core calculations as obtained with the two methods are compared. It is found that the nonorthogonal formu lation predicts equilibrium geometrical parameters in some cases simil arly and, in other cases, slightly better than does the orthogonal one . Based on this observation and on the fact that the nonorthogonal for mulation is exempt from empirical parameters (projection factors), it is concluded that the nonorthogonal formalism represents an appealing alternative in reduced basis frozen-core calculations. (C) 1996 John W iley & Sons, Inc.