Configuration interaction test calculations

Full configuration interaction (Cl) calculations for the shieldings in H2 (50) and BH molecules have demonstrated that the CCSD(T) results are sufficiently close to the full Cl results, so that for practical purposes, the CCSD(T) calculations may be considered as the benchmark calculations for each molecule, against which other methods such as density functional methods may be tested. 

The BH molecule continues to be a favorite subject of theoreticians for testing various types of calculations. It contains only six electrons and, since many others have similarly used the molecule as a test for their calculational methods, it provides a useful reference. Restricted configuration interaction (Cl) calculations, for establishing an ordering scheme for virtual self-consistent field (SCF) or secondary multiconfiguration-SCF orbitals, allow contributions from the entire orbital space to be taken into account [1]. 

Finally, in order to illustrate the role of the 1-MZ purification procedure in improving the approximated 2-RDMs obtained by application of the independent pair model within the framework of the SRH theory, all the different spin-blocks of these matrices were purified. The energy of both the initial (non-purified) and updated (purified) RDMs was calculated. These energies and those corresponding to a full configuration interaction (full Cl) calculation are reported in Table 111. As can be appreciated from this table, the nonpurified energies of all the test systems lie below the full Cl ones while the purified ones lie above and very close to the full Cl ones. 

The long-standing interest of Boyd and his coworkers in radicals and radical ions has led to many papers since 1993 on hyperfine structures. These papers have pushed the conventional multireference configuration interaction methods to the limits of the available computers, tested the predictive ability of various functionals commonly used in DFT calculations, and, among other topics, modeled the effect of a noble gas matrix on the hyperfine structures of radicals. Recent research focused primarily on radicals formed as a consequence of radiation damage to DNA. 

Calculated by a Configuration Interaction Method Using Determinants of Two-Component Molecular Spinors Test Calculations on Rn and T1H. 

As far as standard ab initio calculations go, it appears to me that in the future there will be improvements to the computational techniques, though my guess is that at the moment they are ahead of the gas-phase experimental techniques. An example of such a computational improvement [94] is the frequency-dependent moment method in which hyperpolarizabilities are evaluated from configuration-interaction matrices without diagonalizing them. However, it should be remembered that calculations of high-order properties, such as the ones we have been discussing here, are a very tough test of computational quantum chemistry. 

During the past few years, we have been interested in the calculation and interpretation of transition metal optical spectra conducted in parallel to ongoing experimental studies. To this end, we have developed efficient computer codes to carry out such calculations [45], In particular, a method called spectroscopy oriented configuration interaction (SORCI) was developed [46] and successfully tested in a number of applications [47-57] (for an extensive review on the these developments and their application, see [58]). 

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