An improved externally contracted CI method

An improved externally contracted CI algorithm is presented, aimed at reduc ing correlation energy loss and ensuring high efficiency. Some examples are tested by the new contracted CI method and improvements to various degrees with respect to original externally contracted CI are shown. Moreover, a n ew definition of the 'canonical' order for integrals is suggested which see ms to be more efficient and general for addressing integrals with two or mo re indices. (C) 1999 Elsevier Science B.V. All rights reserved.