# Spin-Network-Scaled MP2 (SNS-MP2) by D. E. Shaw

Code author: D. E. Shaw Research

Section author: Shannon E. Houck

This plugin is an implementation of the SNS-MP2 algorithm developed by McGibbon et. al. [McGibbon:2017:161725]. The SNS-MP2 method uses neural networking to improve the accuracy of MP2 (dfmp2) interaction energies for dimer molecules. The plugin is distributed under the 2-clause BSD license.

## Installation

Binary

- snsmp2 is available as a conda package for Linux and macOS (and Windows, through the Ubuntu shell).

- If using the Psi4conda installer, snsmp2 has already been installed alongside.

- If using the PSI4 conda package, the snsmp2 conda package can be obtained through `conda install snsmp2 -c psi4` or `conda install psi4-rt -c psi4`.

- If using PSI4 built from source, and anaconda or miniconda has already been installed (instructions at Quick Installation), snsmp2 can be obtained through `conda install snsmp2 -c psi4`. Then, hint its location with `PYTHONPATH`.

- To remove a conda installation, `conda remove snsmp2`.

Source

- Download the plugin from the GitHub repository:

      >>> git clone https://github.com/DEShawResearch/sns-mp2

- Once dowloaded, the plugin can be installed as outlined in the documentation:

      >>> cd {top-level-sns-mp2-directory}
      >>> PSI4_PYTHON=$(head $(which psi4) -n 1 | sed -r 's/^.{2}//')
      >>> $PSI4_PYTHON -m pip install .

## Sample Input

A sample input file, adapted from the documentation, is shown below:

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    # Sample SNS-MP2 calculation for two helium atoms

    molecule dimer {
    He 0 0 0
    --
    He 2 0 0
    }

    energy('sns-mp2')

Note that the two monomers are separated by double dashes, indicating that they should be treated as separate molecules. (See sec:analysis-of-intermolecular-interactions for more details on setting up dimer molecules.) This input file can be run in the usual fashion:

    >>> psi4 input.dat