UC Riverside

Molecular aggregates like molecular crystals and clusters find

important applications as pharmaceutical drugs, explosives, organic

semi-conductors, materials for fuel storage, etc. These systems are

dominated by a variety of intermolecular interactions of different

strengths like hydrogen bonding, dispersion, electrostatics and

induction. Traditional classical force field methods for studying the

properties of these chemical systems lack the desirable accuracy for

treatment of these different types of intermolcular interactions,

while efficient treatment with electronic structure methods like

second-order perturbative Moller-Plesset (MP2) and coupled cluster

methods are unaffordable for these large chemical systems. Methods

based on density functional theory (DFT) suffer from their inability

to be systematically improvable. Hence, alternative methods are

desirable for electronic structure quality predictions while being

computationally affordable for these molecular crystals and clusters.

The Hybrid Many-Body Interaction (HMBI) method described in this

dissertation has been developed for studying the properties of these

molecular aggregates. In this method, the system is broken down into

fragments and the most important short-range interactions are treated

using highly accurate electronic structure methods while the less

important but more expensive interfragment interactions are treated

using inexpensive classical force fields. Here, we demonstrate that

the HMBI predictions are electronic structure quality while being

computationally affordable. Moreover, these predictions can be

systematically improved by use of more accurate electronic structure

methods and force fields.

Here, the HMBI method has been employed in predicting the energetics

and structure of molecular crystals and clusters. Some other

capabilities of this method include prediction of the crystal

structure in the presence of external stress, vibrational spectra,

phonon dispersion curves, thermal properties like sublimation heats

and specific heat capacities and elastic constants. We demonstrate

that accurate HMBI predictions of these crystal properties allows for

accurate identification and screening of different crystal polymorphs

which is important in various applications of these materials.