Quantum+Chemistry+FAQ

=Electronic structure theory, covered in a series of snippets and tex documents=

Electron correlation and use of the Dunning correlation consistent basis sets
How many times have you seen this: "... we've performed a fully correlated MP2/CCSD/CCSD(T) calculation using the cc-pVTZ basis." Would it surprise you to learn that such a calculation is actually wrong? Involving core orbitals in correlated methods with the polarized valence triple-zeta basis invites our codes to approximate the correlation energy using core orbitals modeled by a minimal basis. Only the valence orbitals have the additional functions, allowing for a fundamentally more accurate description of the size extent and shape of the orbital due to the external electronic perturbations induced by nearby atoms. If a fully correlated calculation is desired, you should apply a basis set denoted by the presence of the pair of letters, 'CV.' Such a basis set applies additional functions to both core and valence minimal descriptions. The core-valence counterpart of the the cc-pVTZ basis set mentioned above would be cc-pCVTZ. Diffuse functions (prefix aug-) are available too.

Check out the EMSL Basis Set Exchange to gain access to these basis sets!

When should I...

 * ===use a core-valence basis set?===
 * Alkali and alkaline earths! The heavier the atom, the closer in energy the valence and outermost core orbitals.
 * I've found this to be useful for fluoride and I think is ideal for chemically very reactive species
 * ===compute the relativistic corrections?===
 * Technically, we should always consider relativity (spectroscopic accuracy, I've heard it termed)
 * However, the rule of thumb centers on 4th row elements and onwards
 * You will find on EMSL that the transition metals are some of the most heavily parametrized elements in quantum chemistry
 * ===worry about QED?===
 * Similar to relativistic corrections, consider these for heavy elements (6th row onwards)
 * I otherwise have NO experience with these calculations, but I bet you've heard of the Lamb shift.
 * ===account for basis set superposition error (BSSE)?===
 * Briefly, BSSE is a consequence of our finite basis sets and the effect is suppressed by using larger and larger basis sets
 * BSSE affects intermolecular interaction energies and is often applied in geometry optimizations of vdW complexes and subsequent energy calculations
 * Functions from monomer A, artificially stabilize monomer B, and vice versa, resulting in an interaction energy lower than the true value
 * There exist a few ways to deal with BSSE aside from simply using large basis sets (i.e., Dunning triple- or quadruple-zeta quality)
 * Counterpoise (CP)
 * Symmetry adapted perturbation theory (SAPT) -- energy calculation ONLY!
 * Chemical Hamiltonian approach (CHA) -- personally not as familiar with


 * to be continued*

TPP 08/24/2012