Can a Finite Chain of Hydrogen Cyanide Molecules Model a Crystal?**

Abstract

When calculating structural or spectroscopic properties of molecular crystals, the question arises whether it is sufficient to simulate only a single molecule or a small molecular cluster or whether the simulation of the entire crystal is indispensable. In this work we juxtapose calculations on the high-pressure structural properties of the (periodic) HCN crystal and chains of HCN molecules of finite length. We find that, in most cases, the behavior of the crystal can be reproduced by computational methods simulating only around 15 molecules. The pressure-induced lengthening of the CÀ H bond in HCN found in calculations on both the periodic and finite material are explained in terms of orbital interaction. Our results pave the way for a more thorough understanding of high-pressure structural properties of materials and give incentives for the design of materials that expand under pressure. In addition, they shed light on the complementarity between calculations on periodic materials and systems of finite size.

Computational high‐pressure chemistry: Ab initio simulations of atoms, molecules, and extended materials in the gigapascal regime

“…Using X-HCFF and GOSTSHYP it was recently found that simulating a chain with approximately 15-20 HCN molecules reproduces most of the highpressure structural parameters of a HCN crystal occurring along the crystallographic c axis, that is, the direction along which the HCN molecules are aligned in the crystal. 194 A similar comparison has been made using XP-PCM in the case of the realgar crystal. 365 Melicherov a and Marto ň ak studied the polymerization of nitrogen at high pressure and temperature using NPT MD simulations.…”

“…193 In the case of hydrogen cyanide (HCN) chains, a mixed PBC, X-HCFF, and GOSTSHYP study using DFT showed that the pressure-induced strengthening of the interaction between the nitrogen lone electron pair and the carbon-hydrogen σ Ã orbital leads to a lengthening of the carbon-hydrogen bond. 194 As can be seen from these examples, pressure-induced changes in electronic configurations, atomic radii, chemical bonds, and atomic/molecular properties are diverse and, in many cases, system-specific. Studying them is a highly fruitful endeavor, leading to a better understanding of pressure-induced chemistry, while at the same time challenging our understanding of fundamental chemical concepts.…”

“…Therefore, depending on the chemical system under consideration, strengthening intermolecular interactions between functional groups by pressure may either be stabilizing 192 or possibly preconditioning the molecules for polymerization 193 . In the case of hydrogen cyanide (HCN) chains, a mixed PBC, X‐HCFF, and GOSTSHYP study using DFT showed that the pressure‐induced strengthening of the interaction between the nitrogen lone electron pair and the carbon–hydrogen $$$ {\sigma}^{\ast } $$$ orbital leads to a lengthening of the carbon–hydrogen bond 194 …”

Trapping the Transition State in a [2,3]-Sigmatropic Rearrangement by Applying Pressure

“…Several quantum chemical methods for the application of hydrostatic pressure to molecules during geometry optimizations have been reported. − In this paper, we used the extended hydrostatic compression force field (X-HCFF) approach to apply pressure during geometry optimizations and Born–Oppenheimer molecular dynamics (BOMD) simulations, as implemented in the Q-Chem 5.4.2 program package, since this method has been used successfully to predict structural parameters and chemical reactions in the GPa regime . Density functional theory (DFT) , at the B3LYP − /6-31G­(d) level of theory was used as the electronic structure method, and 302 tessellation points per atom as well as a tessellation sphere scaling factor of 1.0 were used in the X-HCFF calculations.…”

“… 43 − 30 In this paper, we used the extended hydrostatic compression force field (X-HCFF) approach 29 to apply pressure during geometry optimizations and Born–Oppenheimer molecular dynamics (BOMD) simulations, as implemented in the Q-Chem 5.4.2 program package, 31 since this method has been used successfully to predict structural parameters and chemical reactions in the GPa regime. 44 Density functional theory (DFT) 32 , 33 at the B3LYP 34 − 36 /6-31G(d) 37 level of theory was used as the electronic structure method, and 302 tessellation points per atom as well as a tessellation sphere scaling factor of 1.0 were used in the X-HCFF calculations. The choice of using this set of X-HCFF parameters was motivated in the original publication.…”