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A. Alenaizan's restricted electrostatic potential (RESP) plugin to Psi4

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Restrained Electrostatic Potential
==================================

## Installation
This RESP code is a plug-in to the Psi4 quantum chemistry package.
To install Psi4, go to the this website: https://admiring-tesla-08529a.netlify.com/installs/latest/ and
choose your preferred option (conda package recommended). If the RESP program was not installed by
default, execute `conda install resp -c psi4`. To test that the RESP plug-in works,
execute `python -c "import resp, sys; sys.exit(resp.test('long'))"`.
Two examples on how to use the plug-in are available in the directory `examples`.

## Code
The following codes are available:
- `resp/espfit.py`: The restrained electrostatic potential (RESP) fitting procedure.
- `resp/driver.py`: Driver of the code.
- `resp/tests/test_resp.py` contains:
   - `test_resp_1`: Example for two-stage charge fitting for one conformer.
   - `test_resp_2`: Example for two-stage charge fitting for two conformers.

Helper programs:
- `resp/vdw_surface.py`
- `resp/stage2_helper.py`

## Examples
There are several examples in `examples` on how to use the code for computing
RESP charges and for enforcing charge constraints

## RESP Method Summary
RESP charges result from fitting the classical electrostatic potential (ESP)
generated by atom-centered point charges to the quantum ESP computed outside
the van der Waals surface of the molecule. The fitted charges are restrained
by a hyperbolic term, which requires an iterative fitting procedure to compute
the charges.

The charges (**q**) are computed by solving the following equation:

**A** **q** = **B**.

The left-hand side contains the information about the classical ESP while the
right-hand side contains information about the quantum ESP.
A hyperbolic restraint term that depends on the charges is added to the diagonal
elements of matrix **A**. The charges and the diagonal elements of **A** are
changed iteratively until the charges converge.

## References:
- [[Bayly:93:10269-10280](https://pubs.acs.org/doi/abs/10.1021/j100142a004)] C. I. Bayly *et. al.* *J. Phys. Chem.* **97**, 10269 (1993)

Please cite this article if you use this program:
- [[Alenaizan:19](https://doi.org/10.1002/qua.26035)] A. Alenaizan, L. A. Burns, C. D. Sherrill, Python implementation of the restrained electrostatic potential charge model. *Int. J. Quantum Chem.*. (2019)

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