DENSITY-FUNCTIONAL THEORY OF MOLECULAR-SOLIDS - LOCAL VERSUS PERIODICEFFECTS IN THE 2-DIMENSIONAL INFINITE HYDROGEN-BONDED SHEET OF FORMAMIDE

The performance of an ab initio computational scheme for molecular cry stals based on density functional theory (DFT) was investigated by com puting several structural and energetic properties of hydrogen bonded infinite chains and of two-dimensional infinite periodic networks of f ormamide. The applied DFT potentials covered a wide range in quality, starting with a simple local exchange (X) without correlation (C), and then gradually introducing C and gradient corrections for both X and C. At the same time, five atomic basis sets of systematically increasi ng size, in the range of DZ to TZ(2df,2pd) were used to construct the Bloch-type crystal orbitals, to optimize the structures, and to extrap olate different physical quantities to the limit of a hypothetical inf inite basis set. Infinite lattice sums were computed by the multipole expansion technique, and basis set superposition errors were (partly) eliminated by the counterpoise method. To be able to assess the accura cy of the theoretical models, the formamide monomer and two different dimers were also investigated using the same methods. Detailed compari sons were made for all models also with recent results obtained by usi ng different orders of many-body perturbation theory. Structural optim izations for the dimers and the infinite crystal demonstrated the impo rtance of gradient terms both for exchange and correlation. For the mo st successful DFT functional, containing the Becke exchange and the Le e-Yang-Parr correlation term, the lengths of the hydrogen bonds, R(HB) , were reduced by 0.16-0.19 Angstrom (depending on the basis set) as c ompared with dimers, due to cooperative interactions in the crystallin e environment. The binding energies were increased typically by 60-70% . The theoretical model explained why the R(HB) values for open-chain dimers become shorter in the crystal than those obtained for the cycli c ones (as opposed to free dimers), correctly predicted changes of bon g lengths in going from the monomer to the crystal, and provided N-H . .. O bond distances and lattice constants reasonably close to experime nts.