Ab initiothermodynamic properties and their uncertainties for crystalline α-methanol

Abstract

To investigate the performance of quasi-harmonic electronic structure methods for modeling molecular crystals at finite temperatures and pressures, thermodynamic properties are calculated for the lowtemperature a polymorph of crystalline methanol. Both density functional theory (DFT) and ab initio wavefunction techniques up to coupled cluster theory with singles, doubles, and perturbative triples (CCSD(T)) are combined with the quasi-harmonic approximation to predict energies, structures, and properties. The accuracy, reliability, and uncertainties of the individual quantum-chemical methods are assessed via detailed comparison of calculated and experimental data on structural properties (density) and thermodynamic properties (isobaric heat capacity). Performance of individual methods is also studied in context of the hierarchy of the quantum-chemical methods. The results indicate that while some properties such as the sublimation enthalpy and thermal expansivity can be modeled fairly well, other properties such as the molar volume and isobaric heat capacities are harder to predict reliably.The errors among the energies, structures, and phonons are closely coupled, and most accurate predictions here appear to arise from fortuitous error compensation among the different contributions.This study highlights how sensitive molecular crystal property predictions can be to the underlying model approximations and the significant challenges inherent in first-principles predictions of solid state structures and thermochemistry.

Ab initioprediction of the polymorph phase diagram for crystalline methanol

“… 44 That value improves upon earlier predictions by up to several kJ mol –1 . 42 , 45 , 46 The predicted change in enthalpy associated with the α → β transition at 157 K and ambient pressure is 0.41 kJ mol –1 , versus 0.64 kJ mol –1 experimentally. 47 The experimentally observed decrease in the isobaric heat capacity across the α–β phase transition 47 is reproduced qualitatively, though the heat capacities themselves are underestimated by 10–20% ( Fig.…”

“…The underestimation of the heat capacity likely stems from a mixture of overestimation of the lattice mode phonon frequencies, the neglect of anharmonicity in the intramolecular modes (particularly methyl rotations), and underestimation of the thermal expansivity. 42 …”

“…As described in our recent study of α methanol, 42 the crystal structures were relaxed using the hybrid many-body interaction (HMBI) fragment approach, 37 employing density-fitted MP2 in the aug-cc-pVTZ basis set 62 (Molpro 2012.1 ( ref. 63 )) for the one-body and short-range two-body terms and the AMOEBA force field 38 (Tinker 6.2 ( ref.…”

Overcoming the difficulties of predicting conformational polymorph energetics in molecular crystals via correlated wavefunction methods

“…For the HMBI fragment calculations employing correlated wave function methods, large basis sets must be used to ensure convergence of the polymorph energetics, as demonstrated in ESI Section S1.2 † and many previous studies. 10,81,82,93,94,116,123,124 Here, MP2 and MP2D monomer and dimer energies at the complete-basis-set (CBS) limit were obtained using a development version of PSI4. 125 The correlation energy was extrapolated 126 to the complete-basis-set (CBS) limit using data from the aug-cc-pVTZ and aug-cc-pVQZ basis sets 127 and combined with HF/aug-cc-pVQZ.…”

Thermal Gradient Approach for the Quasi-harmonic Approximation and Its Application to Improved Treatment of Anisotropic Expansion

“…Quasi-anisotropic QHA (QuasiAniso-QHA): Quasi-anisotropic QHA uses a similar methodology to that implemented in the CRYSTALS program 36 as well variants used by other researchers. 32 The lattice minimum structure is first isotropically expanded and compressed to several volumes of interest. We then perform a volume-constrained optimization of the lattice energy for each volume, and then fit the potential energy with an equation of state and a polynomial fit to each vibrational frequency.…”