# Dual Basis SCF and MP2 Methods

- SCF and correlation calculations with large basis set with many polarized or diffused functions are necessary to achieve chemical accuracy in many cases, while posing challenges because of high computational cost and slow SCF convergence.
- Dual basis methods : 1) A SCF calculation with a relatively small basis set is done; 2) The density matrix in the small basis set is projected onto the large basis set; 3) A single Fock matrix building step in the large basis set is taken and the total energy is improved; 4) The obtained MOs in the large basis set can be used to evaluated the correlation energy at the MP2 level of theory.
- Analytic energy gradients are avaible for dual basis SCF and MP2, so that geometry optimization and AIMD calculations with large basis sets can be done with very high efficiency.

Calculated vibrational absorption spectra for two isomers of the NO^{+}(H2O)_{3} complex from BOMD at the DB-RI-MP2/6-31++G** level of theory

The corresponding experimental vibrational absorption spectra