Theoretical materials design based on quantum mechanical calculations, usually called ab initio simulation or the first principles calculation, is a present day state of art method, which is believed to predict new industrially useful materials without experimental help. However, normally used density functional theory (DFT), which drastically reduces amount of computation from 3n dimension (n=number of electrons) to 3 dimension has two fundamental problems; (1) ground state theory, and (2) electron exchange-correlation functional is not known. Therefore most of the present simulations based on this way use phenomenological parameters to fit to experimental observations (LDA+U, hybrid, etc.). We have indicated how to certify various simulation methods by checking the virial ratio (V/T) to be -2 (ΔE=-ΔT=V/2), which is a necessary condition for any kind of materials composed of nucleus and electrons interacting via Coulomb force in equilibrium phase described by Schroedinger equation. We also invented several new methods to be able to compute necessary physical/chemical properties to design new materials without fitting to experimental results (true ab initio simulation method); (1) GW approximation with all electrons, which supplies absolute energy estimation named TOMBO approach (TOhoku Mixed-Basis Orbitals ab initio simulation package), (2) computing method of van der Waals interaction, which arises from dipole-dipole interaction and is not related to exchange-correlation of electrons, and (3) tracing of chemical reaction by applying time dependent density functional theory (TDDFT). These new methods are applied to design efficient and clean hydrogen storage materials with theoretical certification of numerically estimated results and have power of prediction of new materials before experimental studies.
Virial theorem satisfaction has been tested for several cases of atoms and small molecules and proved some of the frequently used quantum mechanical model calculations such as Heitler-London and Hubbard models are completely wrong; they stabilize the system reducing kinetic energy (disperse electronic cloud) and increasing potential energy, which contradicts the nature (since virial theorem requires potential energy decrement and kinetic energy increment to stabilize the system). This new criterion is applicable to new hydrogen storage material design to certify our theoretical predictions.
To study one of the most important physical properties in semiconductor and insulator, band gap, it is a minimum condition in DFT to add GW approximation. GW calculations have been performed by limited number of research groups over the world because of requested large computer power (O(n6) calculation) but with satisfactory results. However, they used pseudopotential with plane wave expansion of the electronic wavefunciton and cannot estimate absolute energy values of HOMO and LUMO levels. We have invented all electron full potential calculation (TOMBO) with GW approximation to be able to estimate these values in absolute values (ionic potential and electron affinity). We have been successful to compute these values for several important systems and now try to apply to hydrogen storage materials.
Van der Waals force has been one of the most important interactions among inert gas atoms, neutral molecules, polymers, etc. However, up to the present, this fundamental interaction has difficulty to be numerically estimated because of the nature of the force is dipole-dipole interaction which is the results of excitation of objects. We have applied TOMBO approach and TDDFT to estimate successfully van der Waals interaction, which is the main part of the interaction of hydrogen molecule and the storage materials. This part of the present research will be delivered by one of our collaborators (Vladimir Belosludov).
TDDFT is a theoretical method used as a standard to estimate photoreaction cross section in a time zone to reduce amount of computation. We apply TDDFT as a dynamic simulation tool to real time evolution of typical atom-rearrangement systems (as a time dependent Schroedinger equation solver). This new approach has been used to simulate hydrogen molecule dissociation by metallic catalyst (hydrogen molecule spill over, which realizes higher density storage of hydrogens). Our simulation results, for example system with nickel cluster on graphene dissociates hydrogen molecule, indicate lower energy dissociation compared to standard total energy surface calculations.
Our main target for hydrogen storage materials is hydrate clathrate, which is expected to be the cleanest energy carrier, since it stores hydrogens and after releasing them only left is water. We have applied our theoretical methods described above and studied a number of cases to stabilize hydrate clathrates with help gas for efficient hydrogen storage to be able to be used industrially; such as hydrogen car. Especially we apply thermodynamics to estimate P-T phase diagram to serve important information for practical usages. The present method is extensively applied to other gas storages in hydrate clathrates (CO2, CH4, etc.) This part is given by one of our collaborator (Rodion Belosludov).
For future studies we plan to combine all the newly developed methods described above and predict more stable and higher density hydrogen storage materials with theoretical confidence prior to experiment (suggesting experimentalists to follow what we predict).
The author is thankful to the Russian Megagrant Project No.14.B25.31.0030 “New energy technologies and energy carriers” for supporting the present research.