Understanding and controlling polymorphism in molecular solids

the research questions the project is trying to address/the objectives of the project;

Understanding and controlling polymorphism, where molecules crystallise into multiple solid forms, is a major unsolved problem in structural chemistry. While some theoretical insight into polymorphism can be obtained by calculating lattice energies, force fields and Density Functional Theory do not readily decompose energies into different types of interaction. This severely limits the insight available from crystal-structure prediction studies that could otherwise point to new predictive rules or "smarter" screening approaches.
One objective is to identify the pertinent interactions in polymorphic systems by using a recent energy partitioning method called Interacting Quantum Atoms (IQA).
The second objective is to train a machine-learning technique called kriging on these atomic energy contributions to derive an accurate and transferable force field called FFLUX. We will use FFLUX to explore larger problems such as predicting surface termination/reactivity and crystal morphology.
The third objective is to incorporate FFLUX into the UKRI-sponsored software package DL_POLY (Daresbury lab), and to prove that the results obtained by FFLUX are indeed closer to experiment than those produced by classical force fields.
A fourth objective is to automate these processes by using the Cambridge Structural Database to select molecules and identify the major degrees of freedom for building the force field. Our ultimate aim is to provide a comprehensive database of parameters and software to allow crystal engineers to perform "point and click" force field calculations with DFT levels of accuracy.



the approach that will be taken to answer these questions (what the student will actually be doing);
This project builds on the results of the EPSRC Fellowship entitled "Reliable computational prediction of molecular assembly". This work has led to a next-generation in-house force field called FFLUX. This force field is much more realistic than a point-charge based force field such as AMBER. FFLUX also introduces multipole moments, which are essential for accurate electrostatics governing a considerable part of the non-covalent interactions between the typically polar molecules that form the crystals.

The modern IQA method offers a step change in the rigour of atomistic energy analysis. IQA is a parameter-free method that is intuitive but, at the same time, very close to the quantum mechanical character of atoms themselves. When combined with the in-house Relative Energy Gradient (REG) method IQA returns powerful qualitative statements on which atoms govern the behaviour of the overall system and why (i.e. by which type of energy).

The REG method has been developed in the main supervisor's group and comes with an in-house code called REG.py.

The student will be programming in FORTRAN90 and Python, modifying the in-house codes ICHOR, DL_FFLUX and REG.py to interface them with the problem of crystal structure prediction. He will run molecular dynamics simulations using FFLUX, for the first time on crystals. The student will be involved with careful and systematic testing and thus gather unprecedented insight. He will fine-tune the parameterisation of FFLUX. Comparisons with classical force fields will also be made.



the novel engineering and/or physical sciences content of the research (the science that places it within EPSRC's remit).
This project resorts under the Chemical Sciences Grand Challenge of "Directed Assembly of Extended Structures with Targeted Properties (DAESTP)". There is a strong Machine Learning component to this project, and thus overlap with Artificial Intelligence, a popular funding topic. The associated scientific product is called FFLUX, which is a completely new force field, designed by novel principles and encoded as a software package.