Practical quantum mechanics-based fragment methods for predicting molecular crystal properties

Abstract

Significant advances in fragment-based electronic structure methods have created a real alternative to force-field and density functional techniques in condensed-phase problems such as molecular crystals. This perspective article highlights some of the important challenges in modeling molecular crystals and discusses techniques for addressing them. First, we survey recent developments in fragment-based methods for molecular crystals. Second, we use examples from our own recent research on a fragment-based QM/MM method, the hybrid many-body interaction (HMBI) model, to analyze the physical requirements for a practical and effective molecular crystal model chemistry. We demonstrate that it is possible to predict molecular crystal lattice energies to within a couple kJ mol(-1) and lattice parameters to within a few percent in small-molecule crystals. Fragment methods provide a systematically improvable approach to making predictions in the condensed phase, which is critical to making robust predictions regarding the subtle energy differences found in molecular crystals.

Determining the cohesive energy of coronene by dispersion-corrected DFT methods: Periodic boundary conditions vs. molecular pairs

“…molecules or fragments, responsible for their self-assembly, is difficult to evaluate directly and depends on many-body electronic interactions. 57,58 Thus, for computational efficiency and for insightful physical interpretations, it is worthwhile to decompose it into contributions from dimers, trimers, and so on. This leads to an accurate decomposition, known as the many-body expansion…”

Thermal properties of molecular crystals through dispersion-corrected quasi-harmonic ab initio calculations: the case of urea

“…Only in recent years, have different theoretical approaches been devised in order to predict their structural and energetic properties (with the main goal of discriminating between competing polymorphs): from force-field to high-level molecular fragment-based schemes, from periodic dispersion-corrected density functional theory (DFT-D) to periodic many-body wave-function techniques. [10][11][12][13][14][15][16][17][18] However, once a reliable and balanced description of the various chemical interactions has been achieved by means of any of the above-mentioned quantum-chemical methods, the extension of their applicability to more complex properties of technological and industrial relevance, which would greatly increase their predictiveness, such as mechanical, elastic, optical and thermodynamic responses, [19][20][21][22] has to be performed. Apart from the intrinsic high degree of complexity of the required theoretical techniques and algorithms, the main difficulty here is represented by the fact that most of those properties are largely affected by thermal effects, [23][24][25] even at room temperature, such as zero-point energy, harmonic and anharmonic thermal nuclear motion, thermal lattice expansion, etc.…”

Density-functional approach to the three-body dispersion interaction based on the exchange dipole moment

“…7,[10][11][12] Current DFT methods tend to overestimate the two-body dispersion interaction partly due to the omission of the three-body contribution that is often of the opposite sign. 7 The general formulas for the nonadditive threebody dispersion were derived long ago based on perturbation theory and dynamic polarizabilities, [13][14][15] but were not applied in DFT studies of VDW systems until recent years.…”

“…The significance of the non-additive three-body dispersion term is still not quite clear and is being debated. [7][8][9] It was estimated to be as large as 10% of the stabilization energy in some molecular crystals 10 and about 46% of the energy difference between folded and elongated polyalanine decamers. 7 The three-body dispersion contribution was also shown to be crucial to the a) Author to whom correspondence should be addressed.…”