# Selection Method of Basis Set in Quantum Chemistry

The choice of basis set in quantum chemistry is an art. Researchers with rich research experience and quantitative theoretical intuition can choose the most cost-effective basis set for the problem, system, and required accuracy of the research, while beginners are almost always the basis set. The group selection is inappropriate. The purpose of this article is to provide beginners with general guidance on the selection of the basic group.

What basis set should be used? Before considering this problem, the following elements must be clarified and judged comprehensively.
a) What kind of system, how many atoms, and what elements are there
b) What is the problem
c) How high precision is required and what theoretical method is used
d) Acceptable calculation time
e) What program to use

### Dipole moment, polarizability, and first hyperpolarizability

The dipole moment, polarizability, and first hyperpolarizability are the first, second, and third derivatives of the energy to the external electric field, respectively. These three must be calculated with a dispersion function, otherwise the result is useless. And as the order of the derivative increases, the requirements for the dispersion function are getting higher and higher.

Calculating the dipole moment: It is recommended to use def2-TZVPD, if this does not work, it will be reduced to def2-SVPD. pople
aug-cc-pVTZ is also very good for dipole moment calculation.

Calculating the polarization rate: It is strongly recommended to use def2-SVPD. TZVPD. TZVPPD. QZVPD, QZVPPD. The size increases in order, and you can choose according to your calculation ability.

Calculating the first hyperpolarizability: Sadlej HYPOL (CPL,297,391) is recommended, which is specially optimized for calculating the hyperpolarizability.

### Vibration frequency, VCD, Raman calculation

For large systems, 6-31G* is actually sufficient for this type of task base group, and TZVP is sufficient at most. It can't be improved any more, and it is a waste of time. Generally speaking, these tasks are calculated based on resonance approximation, and frequency correction factors are generally used to correct the systematic errors of resonance approximation and theoretical methods.

All-electron relativistic calculations

All-electronic relativity calculations need to use DKH, ZORA. X2C and other Hamiltonian considering relativistic effects. At this time, it is necessary to use the basis set that optimizes the shrinkage method for relativity calculations, such as cc-pVnZ-DK series, SARC, etc., Or simply use a completely unshrinkable basis set such as UGBS.

### Wave function analysis

For wave function analysis based on real space functions, such as discussing electron density, ELF, electrostatic potential, AIM, RDG analysis, etc., the basis set 6-31G*- is generally sufficient to get qualitatively correct results, and 6- 311G** or TZVP is more than enough. If it is an anionic system, adding layers of s, p angular momentum dispersion functions is also sufficient. It is useless to increase the basis set, because these real space functions will soon increase with the basis set. convergence.

### Solvation energy

To calculate the solvation energy using the implicit solvent model, you must use the special parameterized level of the solvent model. For example, the best level for SMD parameterization is M05-2X/6-31G*. Therefore, it is recommended to calculate the dissolution free energy in combination with SMD at the level of M05-2X/6-31G*.

### NMR calculation

It is strongly recommended to use pcSseg series basis set (JCTC,11,132) for NMR calculation.

* For Research Use Only.
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