Computational Chemistry

Chapter 8

Some “Special” Topics: Solvation, Singlet

Diradicals, A Note on Heavy Atoms and

Transition Metals

Chapters 1 to 7: (a) addressed molecules as isolated entities, without reference to their

surroundings (except for the water dimer); (b) concentrated on calculations by relatively

“automatic” model chemistries; and (c) used mainly organic molecules as illustrations.

This chapter to some extent redresses these constraints.

AbstractFor some purposes solution-phase computations are necessary, e.g. for
understanding certain reactions, and for the prediction of pKain solution. For
introducing the effects of solvation there are two methodologies (and a hybrid of
these two): explicit solvation and continuum solvation.
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 carefully chosen set of molecular
orbitals.
For systems with heavy atoms we often employ pseudopotential basis sets
(frequently relativistic), which reduce the computational burden of large numbers
of electrons. Transition metals present problems beyond those of main-group heavy
atoms: not only can relativistic effects be significant, but electron d- or f-levels,
variably perturbed by ligands, make possible several electronic states. DFT calcu-
lations, with pseudopotentials, are the standard approach for computations on such
compounds.

8.1 Solvation.................................................................

Nature abhors a vacuum

–A dictum of Aristotelian physics

E.G. Lewars,Computational Chemistry,
DOI 10.1007/978-90-481-3862-3_8,#Springer ScienceþBusiness Media B.V. 2011

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