THE CONFIGURATION-DRIVEN APPROACH FOR MULTIREFERENCE CONFIGURATION INTERACTION CALCULATIONS
The rationale for carrying out multireference single- and double-excitation configuration interaction (CI) calculations to obtain highly correlated electronic wave-functions for atoms and molecules is discussed. A detailed description of computational algorithms for implementing a configuration-driven approach for this type of theoretical treatment is given, with emphasis on the use of CI tables to enable an efficient evaluation of Hamiltonian matrix elements between Slater determinants spanning configurations with many open shells. These techniques have recently been incorporated in a Table-Direct CI program version (TD-CI) which minimizes I/O operations and thereby greatly expands the range of applicability of the configuration-driven approach. With the use of improved methods for recognition of excitation relationships between configurations it has become possible in the current implementation to solve secular equations of order exceeding one million randomly selected linear combinations of Slater determinants which are eigenfunctions of the S2 operator. Finally, timing results are presented for a number of test examples and a survey of possible future extensions of these methods is undertaken.
