Basis Set Model chemistry theoretical method and basis

Basis Set

Model chemistry: theoretical method and basis set HF MP 2 CCSD(T) CCSDT … Minimal … Splitvalence … Polarized … Diffuse … High angular momentum … … ∞ … … … Full CI … … … Exact solution Goal: select the most accurate calculation that is computationally feasible for a given molecular system

Model Chemistries - three areas of consideration • Basis sets • Theoretical methods

Basis set • Basis functions approximate orbitals of atoms in molecule • Linear combination of basis functions approximates total electronic wavefunction • Basis functions are linear combinations of gaussian functions – Contracted gaussians – Primitive gaussians

STOs v. GTOs • Slater-type orbitals (J. C. Slater) – Represent electron density well in valence region and beyond (not so well near nucleus) – Evaluating these integrals is difficult • Gaussian-type orbitals (F. Boys) – Easier to evaluate integrals, but don’t represent electron density well – Overcome this by using linear combination of GTOs

Minimal basis set • One basis function for every atomic orbital required to describe the free atom • Most-common: STO-3 G • Linear combination of 3 Gaussian-type orbitals fitted to one Slater-type orbital • CH 4: H(1 s); C(1 s, 2 px, 2 py, 2 pz)

More basis functions per atom • Split valence basis sets • Double-zeta: 2 “sizes” of basis functions for each valence atomic orbital – 3 -21 G CH 4: H(1 s, 1 s'), C(1 s, 2 s', 2 px, 2 py, 2 pz, 2 px', 2 py', 2 pz') • Triple-zeta: 3 “sizes” of basis functions for each valence atomic orbital – 6 -311 G CH 4: H(1 s, 1 s''), C(1 s, 2 s', 2 s'', 2 px, 2 py, 2 pz, 2 px', 2 py', 2 pz', 2 px'', 2 py'', 2 pz'')

More basis functions per atom • Split valence basis sets • Triple-zeta: • Double-zeta:

Total 36 22

Total 42 22

Ways to increase a basis set • Add more basis functions per atom – allow orbitals to “change size” • Add polarization functions – allow orbitals to “change shape” • Add diffuse functions for electrons with large radial extent • Add high angular momentum functions

Add polarization functions • Allow orbitals to change shape – Add p orbitals to H – Add d orbitals to 2 nd row atoms – Add f orbitals to transition metals • 6 -31 G(d) - d functions per heavy atoms – Also denoted: 6 -31 G* • 6 -31 G(d, p) - d functions per heavy atoms and p functions to H atoms – Also deonoted: 6 -31 G**

Add diffuse functions • “Large” s and p orbitals for “diffuse electrons” – Lone pairs, anions, excited states, etc. • 6 -31+G - diffuse functions per heavy atom • 6 -31++G - diffuse functions both per heavy atom and per H atom

High angular momentum functions • “Custom-made” basis sets • 6 -31 G(2 d) - 2 d functions per heavy atom • 6 -311++G(3 df, 3 pd) – Triple-zeta valence – Diffuse functions on heavy atoms, H atoms – 3 d, 1 f functions per heavy atom; 3 p, 1 d functions per H atom

Minimal basis sets A common naming convention for minimal basis sets is STO-XG, where X is an integer. This X value represents the number of Gaussian primitive functions comprising a single basis function. In these basis sets, the same number of Gaussian primitives comprise core and valence orbitals. Minimal basis sets typically give rough results that are insufficient for research-quality publication, but are much cheaper than their larger counterparts. Here is a list of commonly used minimal basis sets: STO-2 G STO-3 G STO-6 G STO-3 G* - Polarized version of STO-3 G

Split-valence basis sets During most molecular bonding, it is the valence electrons which principally take part in the bonding. In recognition of this fact, it is common to represent valence orbitals by more than one basis function, (each of which can in turn be composed of a fixed linear combination of primitive Gaussian functions). The notation for these split-valence basis sets is typically X-YZg. In this case, X represents the number primitive Gaussians comprising each core atomic orbital basis function. The Y and Z indicate that the valence orbitals are composed of two basis functions each Here is a list of commonly used split-valence basis sets: 3 -21 g* - Polarized 3 -21+g - Diffuse functions 3 -21+g* - With polarization and diffuse functions 6 -31 g* 6 -31+g* 6 -31 g(3 df, 3 pd) 6 -311 g* 6 -311+g* SV(P) SVP

Double, triple, quadruple zeta basis sets Basis sets in which there are multiple basis functions corresponding to each atomic orbital, including both valence orbitals and core orbitals or just the valence orbitals, are called double, triple, or quadruple-zeta basis sets. Here is a list of commonly used multiple zeta basis sets: cc-p. VDZ - Double-zeta cc-p. VTZ - Triple-zeta cc-p. VQZ - Quadruple-zeta cc-p. V 5 Z - Quintuple-zeta, etc. aug-cc-p. VDZ, etc. - Augmented versions of the preceding basis sets with added diffuse functions TZVPP - Triple-zeta QZVPP - Quadruple-zeta