ON THE TEST OF DIFFERENT ATOMIC EXCHANGE FUNCTIONALS

The yet unknown exchange-correlation energy functional and the lack of a systematic way to find such a functional has led to propose differe nt expressions for exchange, on the one hand, and correlation on the o ther, by several authors. A common factor in the design of exchange fu nctionals is to reproduce different kinds of energies rather than the important atomic shell structure. We present a strategy for testing th e behavior of exchange functionals based on the reproduction of atomic shell structure. A variety of well-known exchange functionals are tes ted on He (closed shell), Li (open shell), and Be (closed subshell). I mportant differences are found among them. Finally, we propose an exch ange energy functional that includes a Becke-88-type expression and, i n addition, a nongradient correction term that leads to improved shell structure behavior. (C) 1995 John Wiley & Sons, Inc.