### Correlation energy extrapolation by many-body expansion

Accounting for electron correlation is required for high accuracy calculations of molecular energies. The full configuration interaction (CI) approach can fully capture the electron correlation within a given basis, but it does so at a computational expense that is impractical for all but the smallest chemical systems. In this work, a new methodology is presented to approximate configuration interaction calculations at a reduced computational expense and memory requirement, namely, the correlation energy extrapolation by many-body expansion (CEEMBE). This method combines a MBE approximation of the CI energy with an extrapolated correction obtained from CI calculations using subsets of the virtual orbitals. The extrapolation approach is inspired by, and analogous to, the method of correlation energy extrapolation by intrinsic scaling. Benchmark calculations of the new method are performed on diatomic fluorine and ozone. Finally, the method consistently achieves agreement with CI calculations to within a few mhartree and often achieves agreement to within ~1 millihartree or less, while requiring significantly less computational resources.

- Publication Date:

- Report Number(s):
- IS-J-9215

Journal ID: ISSN 1089-5639

- Grant/Contract Number:
- AC02-07CH11358

- Type:
- Accepted Manuscript

- Journal Name:
- Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory

- Additional Journal Information:
- Journal Volume: 121; Journal Issue: 4; Journal ID: ISSN 1089-5639

- Publisher:
- American Chemical Society

- Research Org:
- Ames Laboratory (AMES), Ames, IA (United States)

- Sponsoring Org:
- USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)

- Country of Publication:
- United States

- Language:
- English

- Subject:
- 37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY

- OSTI Identifier:
- 1347403

```
Boschen, Jeffery S., Theis, Daniel, Ruedenberg, Klaus, and Windus, Theresa L..
```*Correlation energy extrapolation by many-body expansion*. United States: N. p.,
Web. doi:10.1021/acs.jpca.6b10953.

```
Boschen, Jeffery S., Theis, Daniel, Ruedenberg, Klaus, & Windus, Theresa L..
```*Correlation energy extrapolation by many-body expansion*. United States. doi:10.1021/acs.jpca.6b10953.

```
Boschen, Jeffery S., Theis, Daniel, Ruedenberg, Klaus, and Windus, Theresa L.. 2017.
"Correlation energy extrapolation by many-body expansion". United States.
doi:10.1021/acs.jpca.6b10953. https://www.osti.gov/servlets/purl/1347403.
```

```
@article{osti_1347403,
```

title = {Correlation energy extrapolation by many-body expansion},

author = {Boschen, Jeffery S. and Theis, Daniel and Ruedenberg, Klaus and Windus, Theresa L.},

abstractNote = {Accounting for electron correlation is required for high accuracy calculations of molecular energies. The full configuration interaction (CI) approach can fully capture the electron correlation within a given basis, but it does so at a computational expense that is impractical for all but the smallest chemical systems. In this work, a new methodology is presented to approximate configuration interaction calculations at a reduced computational expense and memory requirement, namely, the correlation energy extrapolation by many-body expansion (CEEMBE). This method combines a MBE approximation of the CI energy with an extrapolated correction obtained from CI calculations using subsets of the virtual orbitals. The extrapolation approach is inspired by, and analogous to, the method of correlation energy extrapolation by intrinsic scaling. Benchmark calculations of the new method are performed on diatomic fluorine and ozone. Finally, the method consistently achieves agreement with CI calculations to within a few mhartree and often achieves agreement to within ~1 millihartree or less, while requiring significantly less computational resources.},

doi = {10.1021/acs.jpca.6b10953},

journal = {Journal of Physical Chemistry. A, Molecules, Spectroscopy, Kinetics, Environment, and General Theory},

number = 4,

volume = 121,

place = {United States},

year = {2017},

month = {1}

}