Electron Correlation from Molecules to Materials

Abstract

In the treatment of electron correlation, ab initio quantum chemical meth­ ods occupy a central role. However, when compared with the approaches used in some areas of physics, particularly with reference to strong correlation, there seems to be important differences. In this chapter, I plan to discuss the dif­ ferent philosophy, access where some limitations are, and suggest some ways to overcome these limitations to the mutual benefit of all aspects of electronic structure theory. Electron correlation from the viewpoint of quantum chemistry introduces two critical approximations: the basis set and the extent of the configura­ tion space that can be included in the correlated treatment. Coupled cluster theory[l, 2, 3, 4] has greatly aided in solving the latter problem, but the basis set problem persists. Today, the CC approach can provide energies ranging from 0.001 to 100 eV for a wealth of chemical and spectroscopic problems.[5, 6] How­ ever, there are still situations that are not accessible. Some of these occur when stronger correlation effects are encountered, while others require eliminating the basis set error. Some ideas are discussed. The relationship of cou'pled-cluster theory to density functional theory is also considered, providing an ab initio DFT framework that should offer converging exchange-correlation potentials and resultant approximations for electronic properties. In addition, coupled­ cluster theory offers the natural vehicle for transferring first-principle electronic structure information into materials modeling, as it provides a rigorous basis for