(and a hybrid of these two): explicit solvation, that is, putting individual solvent
molecules into the system, and continuum solvation, representing the solvent as an
appropriately parameterized continuous medium. Although for some purposes
explicit solvation is needed, particularly where solvent molecules participate in a
reaction, continuum methods are more widely used.
Some molecular species are not calculated properly by straightforward model
chemistries; these include singlet diradicals and some excited state species. For
these the standard method is the complete active space approach, CAS (CASSCF,
complete active space SCF). This is a limited version of configuration interaction,
in which electrons are promoted from and to a limited, carefully chosen set of
molecular orbitals. CASSCF calculations require care in choosing these orbitals
and in judging the reliability of the results (see e.g.Singlet Diradicals,Harder
Questions, Questions 3 and 4).
Calculations on systems with heavy atoms often employ pseudopotential basis
sets, which reduce the computational burden that large numbers of electrons would
present, by avoiding explicit treatment of inner electrons. These basis sets are
frequently relativistic, taking into account the effect on chemical properties of
electrons moving at a significant fraction of the speed of light. Transition metals
present problems beyond those of main-group heavy atoms: not only can relativistic
effects be significant (in the heavier elements), but near-lying electron d- or
f-levels, variably perturbed by various ligands, make possible a variety of electronic
states. Although beyond the first transition metal row ab initio (i.e. wavefunction)
methods have been used, less demanding DFT calculations, with pseudopotentials,
are the standard approach for computations on such compounds.
References....................................................................
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