# Interaction energy models based on the denstiy

**Research Group:**Centre for Condensed Matter and Material Physics

**Number of Students:**1

**Length of Study in Years: **3-4

**Full-time Project: **yes

**Funding: **

** Project Description: **

Intermolecular interaction models form the corner stone of most simulations of the condensed phase. But most of these models are simple and often very inaccurate. Many simulations are performed with Lennard-Jones models, but we have known for decades that these are just too simple and miss out on a lot of detail that is necessary if we wish to make our simulations more predictive. We can develop more accurate models based on ab initio data using methods such as SAPT(DFT) (a symmetry-adapted perturbation theory based on density functional theory), but these are not easy to make and generally incur a considerable computational cost in their development.

Recently, my group, in collaboration with groups in Cambridge and at the University of Wisconsin-Madison in the US, have developed a new methodology to derive intermolecular potentials with only a relatively small amount of ab initio data. In these new methods, most of the terms in an interaction potential models are derived

directly from the electronic charge density, and fitting is kept to a bare minimum. The biggest advantage of this approach is convenience: accurate potentials can be derived in days as opposed to months and years with the old methods. Additionally, the parameters are physical: they mean something and are directly related to the chemical environment of the atoms in the interacting molecules. Finally, these potentials are transferable: the potential parameters for the interaction of A..B can be derived from those obtained from the interactions of A..A and B..B.

In this project, we will extend these methods in a number of important ways:

1) Attempt to eliminate all of the fitting but deriving more or all of the residual parameters from the density.

2) Account for the fact that atoms in a molecule are not spherical: they have a shape that influences the intermolecular interactions.

3) Extend these potentials to the case when the molecules have flexible conformations.

The project will involve a deep understanding of electronic structure methods, in particular, the theory of intermolecular interactions through perturbation theory. It will also require a considerable

amount of programming in Fortran90 and Python, and it is very likely that simulations will need to be performed using codes like DL_POLY and OpenMM.

Candidates need to be strong, or to have a strong desire to learn the required theory and programming techniques. These methods will be coded in the CamCASP program. Candidates will also need to be keen to implement parallel algorithms to best utilise multi-processor computers.

Learning outcomes:

A successful candidate will expect to develop a strong theoretical background in the methods used for intermolecular interactions, in simulation techniques, and programming. Additionally, the candidate will be prepared for further research in a number of fields from this fundamental work on intermolecular potentials. Experience with using simulation codes like OpenMM will place the candidate on a competitive footing for jobs in both academia and industry.

Relevant publications:

SAPT(DFT):

* A. J. Misquitta, R. Podeszwa, B. Jeziorski, and K. Szalewicz, ``Intermolecular potentials based on symmetry-adapted perturbation theory with dispersion energies from time-dependent density functional calculations.'', J. Chem. Phys., 123, 214103-14 (2005).

* A. J. Misquitta, B. Jeziorski, and K. Szalewicz, ``Dispersion energy from density-functional theory description of monomers'', Phys. Rev. Lett. 91, 033201-4 (2003).

Potentials:

* A. J. Misquitta and A. J. Stone, "Ab Initio Atom–Atom Potentials Using CamCASP: Theory and Application to Many-Body Models for the Pyridine Dimer", J. Comp. Theor. Chem., 12, 4184-4208 (2016).

* Mary J. Van Vleet, Alston J. Misquitta, Anthony J. Stone, and J. R. Schmidt, "Beyond Born–Mayer: Improved Models for Short-Range Repulsion in ab Initio Force Fields", J. Comp. Theor. Chem., 12, 3851–3870 (2016).

* A. J. Misquitta, G. W. A. Welch, A. J. Stone and S. L. Price, ``A first principles prediction of the crystal structure of C6Br2ClFH2'', Chem. Phys. Lett. 456, 105-109 (2008).

* T. S. Totton, A. J. Misquitta and M. Kraft ``A first principles development of a general anisotropic potential for polycyclic aromatic hydrocarbons'', J. Comp. Theor. Chem., 6, 683-695 (2010).

**Requirements: **

* A keen mathematical background

* A good knowledge or strong interest in programming in Fortran 90 and Python.

* Ability to learn new theoretical methods quickly.

**SPA Academics: Alston Misquitta**