Theoretical model chemistry wavefunction

Philosophically and mathematically, the NPA methods are wavefunction-oriented, based on variational and perturbation-theoretic use of Hilbert space partitioning to define model chemistry constructs. For a system described by Schrodinger s equation, Hit = E r, it is advantageous to formulate a physical model in terms of a well-defined model Hamiltonian such that standard perturbation theory can be used to assess the corrections to the model, to determine which of several alternative model is uniquely best (gives most rapid... 

The power of theoretical chemistry to address an ever widening array of chemical problems with increasing accuracy demands that equally powerful interpretation methods be developed so that the highly accurate calculations may be placed in a context that is understandable by non-theorists. A subset of methods that aspire to this goal have been summarized here. New methods that are especially applicable to the most sophisticated wavefunctions now available are already under construction. This parallel development of sophisticated theoretical models and the methods needed to interpret them is indeed an exciting prospect. 

The theoretical chemistry community developed density functional theory for finite molecular systems which involve molecules and cluster models that describe the catalytic systems. They use the same constructs used in many ab initio wavefunction methods, i.e. Gaussian or Slater basis sets. The solid-state physics community, on the other hand, developed density functional theory to describe bulk solid-state systems and infinite surfaces by using a supercell approach along with periodic basis functions, i.e. plane waves . Nearly all of our discussion has focused on finite molecular systems. In the next section we will describe in more detail infinite periodic systems. 

There are many ways to compute the ground-state wavefunction Tq, such as configuration-interaction techniques and coupled-cluster methods, widely studied by the ab-initio theoretical-chemistry community. These methods however show a unfavorable scaling with the number of electrons and still cannot be applied to model large systems of increasing interest (e.g. in nano-bio-science). The main limitation is that these methods focus on the many-electron wavefunction T which contains N variables. 

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