# Mp2PairEnergies

## 1.Brief description

Mp2PairEnergies is a matrix $$\epsilon_{ij}$$ of dimensions $$N_{\rm o} \times N_{\rm o}$$. $$\epsilon_{ij}$$ is the electron-pair resolved MP2 correlation energy in the complete basis set limig

$$\epsilon_{ij} = \sum_{AB} \frac{ V^{AB}_{ij}(2V^{ij}_{AB}-V^{ji}_{AB}) } { \varepsilon_i + \varepsilon_j - \varepsilon_A - \varepsilon_B },$$

where $$A$$ and $$B$$ refer to a virtual orbitals in a basis set that can be considered complete. $$\epsilon_{ij}$$ is used by the BasisSetCorrection algorithm and in the context of the theory CCSD(T*) (Irmler, Gallo, and Grüneis 2021). Mp2PairEnergies needs to be provided by an interfaced electronic structure theory package and can be read in using the Read algorithm.

- name: Read
in:
fileName: "Mp2PairEnergies.yaml"
out:
tensor: Mp2PairEnergies


## 2.Specification

An example Mp2PairEnergies.yaml file is given below.

version: 100
type: Tensor
scalarType: Real64
dimensions:
- length:     16
type: State
- length:     16
type: State
elements:
type: TextFile
unit: 0.03674932217563878       # = (Eh/eV)


## 3.Literature

Irmler, Andreas, Alejandro Gallo, and Andreas Grüneis. 2021. “Focal-Point Approach with Pair-Specific Cusp Correction for Coupled-Cluster Theory.” The Journal of Chemical Physics 154 (June): 234103. doi:10.1063/5.0050054.

Created: 2022-09-19 Mon 15:00