Empirical Van Der Waals Correction Research Articles

A six-dimensional potential energy surface for Ru(0001)(2×2):CO.

We present a new global ground state potential energy surface (PES) for carbon monoxide at a coverage of 1/4, on a rigid Ru(0001) surface [Ru(0001)(2×2):CO]. All six adsorbate degrees of freedom are considered. For constructing the PES, we make use of more than 90,000 points calculated with periodic density functional theory using the RPBE exchange-correlation functional and an empirical van der Waals correction. These points are used for interpolation, utilizing a symmetry-adapted corrugation reducing procedure (CRP). Three different interpolation schemes with increasing accuracy have been realized, giving rise to three flavours of the CRP PES. The CRP PES yields in agreement with the DFT reference and experiments, the atop position of CO to be the most stable adsorption geometry, for the most accurate interpolation with an adsorption energy of 1.69 eV. The CRP PES shows that diffusion parallel to the surface is hindered by a barrier of 430 meV, and that dissociation is facilitated but still activated. As a first "real" application and further test of the new potential, the six-dimensional vibrational Schrödinger equation is solved variationally to arrive at fully coupled, anharmonic frequencies and vibrational wavefunctions for the vibrating, adsorbed CO molecule. Good agreement with experiment is found also here. Being analytical, the new PES opens an efficient way towards multidimensional dynamics.

Cooperativity in the stacking of benzene-1,3,5-tricarboxamide: The role of dispersion

Quantum chemical calculations have been carried out to study the stacking and cooperativity in oligomers of benzene-1,3,5-tricarboxamide (BTA). Comparison between density functional theory (DFT) and SCS-MP2/cc-PVTZ level calculations for a dimer indicates the significant contribution of dispersive interactions in this system. Oligomers up to a decamer have thus been studied with DFT augmented by empirical van der Waals corrections. The stabilisation energy of the stack improves with oligomer size and a converged value of −27.1 kcal/mol is estimated. The presence of cooperativity in the system is reflected in a variety of structural parameters of the stack.

Equations of state for energetic materials from density functional theory with van der Waals, thermal, and zero-point energy corrections

It is shown that the introduction of zero-point energy and thermal effects to density functional theory with an empirical van der Waals correction results in a significant improvement in the prediction of equilibrium volumes and isothermal equations of state for hydrostatic compressions of energetic materials at nonzero temperatures. This method can be used to predict the thermophysical properties of these materials for a wide range of pressures and temperatures.

Hydrostatic and uniaxial compression studies of 1,3,5-triamino- 2,4,6-trinitrobenzene using density functional theory with van der Waals correction

Hydrostatic and uniaxial compressions of 1,3,5-triamino-2,4,6-trinitrobenzene were investigated using first-principles density functional theory with an empirical van der Waals correction. The equilibrium structural and elastic properties and the hydrostatic equation of state are in good agreement with available experimental data. Physical properties such as the principal stresses, shear stresses, band gap, and the change in energy per atom as a function of compression ratio V/V0 in the directions normal to the (100), (010), (001),(110), (101), (011), and (111) crystallographic planes were calculated, showing highly anisotropic behavior under uniaxial compressions.

Theoretical Study of the Hydrolysis of Pentameric Aluminum Complexes

Static quantum chemical calculations and Car−Parrinello molecular dynamics (CPMD) simulations were used to investigate the structural characteristics and the stability of pentameric aluminum clusters in both gas phase and aquatic environments. The accuracy of several generalized gradient approximation (GGA) and hybrid exchange-correlation functionals were tested with and without empirical van der Waals corrections to ensure the accuracy of the selected methods. Conformational analysis was performed for experimentally detected (electrospray ionization mass spectrometry, ESI MS) structural isomers of cationic [Al5O6H2Cl4]+, [Al5O7H4Cl4]+, [Al5O8H6Cl4]+, and [Al5O9H8Cl4]+ complexes. Conductor-like screening model (COSMO) was used to investigate the stability of the gas phase optimized structures in aquatic environments. Four of the main pentameric aluminum complexes were then selected for the CPMD investigation. The effect of the long-range empirical vdW corrections (-D) was also tested employing two identic...

Prediction of Isothermal Equation of State of an Explosive Nitrate Ester by van der Waals Density Functional Theory

A new energetic material, nitrate ester 1 (NEST-1), has shown promise as a powerful, technologically attractive explosive. Its physical properties under compression, however, are currently unknown. Accurate density functional calculations together with a reliable empirical van der Waals correction are employed to predict the isothermal hydrostatic equation of state for this material prior to any known experimental results. The accuracy of results obtained from this approach was tested against experimentally known NEST-1 equilibrium properties and found to be excellent.

Self-Assembly of Diacid Molecules: A Theoretical Approach of Molecular Interactions

Dicarboxylic acids have been recently found to provide a significant improvement of the mechanical properties of hard materials such as bitumen when added at low concentration. To understand such properties, periodic DFT calculations combined with empirical van der Waals corrections have been performed on crystals of dicarboxylic acid molecules with various chain lengths. Our calculations allow us to understand the interactions governing the cohesive energy of the crystal and to qualitatively correlate diacid transition phase properties (fusion, vaporization, and sublimation) to their behavior when mixed in bitumen. The molecules are distorted because of structural constraints in the crystal packing and that distortion is found to be larger for odd diacids than for even diacids, explaining the usual odd−even alternation of their properties related to the solid state. In particular, the relative fusion temperature of diacids as a function of chain length is dominated by distortion effects. In contrast, mos...

Importance of van der Waals Interactions in Liquid Water

We present ab initio molecular dynamics studies on liquid water using density functional theory in conjunction with either dispersion-corrected atom-centered potentials or empirical van der Waals corrections. Our results show that improving the description of van der Waals interactions in DFT-GGA leads to a softening of liquid water's structure with higher mobility. The results obtained with dispersion-corrected atom-centered potentials are especially encouraging. In particular, the radial distribution functions are in better agreement with experiment, and the self-diffusion coefficient increases by more than three-fold compared with the one predicted by the BLYP functional. This work demonstrates that van der Waals interactions are essential in fine-tuning both structural and dynamical properties of liquid water.

Tailor-Made Force Fields for Crystal-Structure Prediction

A general procedure is presented to derive a complete set of force-field parameters for flexible molecules in the crystalline state on a case-by-case basis. The force-field parameters are fitted to the electrostatic potential as well as to accurate energies and forces generated by means of a hybrid method that combines solid-state density functional theory (DFT) calculations with an empirical van der Waals correction. All DFT calculations are carried out with the VASP program. The mathematical structure of the force field, the generation of reference data, the choice of the figure of merit, the optimization algorithm, and the parameter-refinement strategy are discussed in detail. The approach is applied to cyclohexane-1,4-dione, a small flexible ring. The tailor-made force field obtained for cyclohexane-1,4-dione is used to search for low-energy crystal packings in all 230 space groups with one molecule per asymmetric unit, and the most stable crystal structures are reoptimized in a second step with the hybrid method. The experimental crystal structure is found as the most stable predicted crystal structure both with the tailor-made force field and the hybrid method. The same methodology has also been applied successfully to the four compounds of the fourth CCDC blind test on crystal-structure prediction. For the five aforementioned compounds, the root-mean-square deviations between lattice energies calculated with the tailor-made force fields and the hybrid method range from 0.024 to 0.053 kcal/mol per atom around an average value of 0.034 kcal/mol per atom.

Energy Ranking of Molecular Crystals Using Density Functional Theory Calculations and an Empirical van der Waals Correction

By combination of high level density functional theory (DFT) calculations with an empirical van der Waals correction, a hybrid method has been designed and parametrized that provides unprecedented accuracy for the structure optimization and the energy ranking of molecular crystals. All DFT calculations are carried out using the VASP program. The van der Waals correction is expressed as the sum over atom-atom pair potentials with each pair potential for two atoms A and B being the product of an asymptotic C(6,A,B)/r(6) term and a damping function d(A,B)(r). Empirical parameters are provided for the elements H, C, N, O, F, Cl, and S. Following Wu and Yang, the C(6) coefficients have been determined by least-squares fitting to molecular C(6) coefficients derived by Meath and co-workers from dipole oscillator strength distributions. The damping functions d(A,B)(r) guarantee the crossover from the asymptotic C(6,A,B)/r(6) behavior at large interatomic distances to a constant interaction energy at short distances. The careful parametrization of the damping functions is of crucial importance to obtain the correct balance between the DFT part of the lattice energy and the contribution from the empirical van der Waals correction. The damping functions have been adjusted to yield the best possible agreement between the unit cells of a set of experimental low temperature crystal structures and their counterparts obtained by lattice energy optimization using the hybrid method. On average, the experimental and the calculated unit cell lengths deviate by 1%. To assess the performance of the hybrid method with respect to the lattice energy ranking of molecular crystals, various crystal packings of ethane, ethylene, acetylene, methanol, acetic acid, and urea have been generated with Accelrys' Polymorph Predictor in a first step and optimized with the hybrid method in a second step. In five out of six cases, the experimentally observed low-temperature crystal structure corresponds to the most stable calculated structure.