Some “Special” Topics: (8.1) Solvation, (8.2) Singlet Diradicals, and (8.3) a Note on Heavy Atoms and Transition Metals

For some purposes solution-phase computations are necessary, e.g., for understanding certain reactions, and for the accurate prediction of pKa in solution. For introducing the effects of solvation there are two methodologies (and hybrids of these two): microsolvation or 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. Also, nearly degenerate s and d levels can cause convergence problems. DFT calculations, with pseudopotentials, are the standard approach for computation on such compounds.