## Petersburg Nuclear Physics Institute (PNPI)## Advanced development division (ADD)## Quantum Physics and Chemistry Department (QPCD) |

The main direction of our work is the development of electronic-structure calculation methods for molecules and materials containing heavy elements. The aim is to elaborate technique and computer codes which allow one to perform very accurate calculations providing lowest computational cost. This can be achieved using “two-step” approaches, in which the calculation of the electronic structure of molecules with heavy atoms (including actinides, lanthanides and heavy transition metals) and their physical-chemical properties is divided into two sequential calculations: first, in the valence region of a chemical compound with using precise relativistic pseudopotentials (see below), and then, in the cores of heavy atoms using procedures for restoration (reconstructing) there the four-component wave function (see below).

This activity was initiated by theoretical developments of Professor Leonty N. Labzovskii and by the experimental studies of the non-conservation of time invariance (T) and spatial parity (P), including searches for the hypothetical “electron electric dipole moment” (eEDM) and T,P-odd properties of nuclei in such molecules as PbF, HgF, YbF, and TlF. The development of two-step computational methods was started in 1983 with PhD work of Anatoly V. Titov. Thesis "*Effective Potentials and Generalized Brillouin Theorem for electronic states of molecules*" was defended in 1986. In 1996, PhD *thesis "Development of Relativistic Effective Core Potential method*" ( __ pdf-file__ in Russian) was defended by Nikolai S. Mosyagin, were topics related to building the generalized relativistic effective core potentials (GRECPs) for heavy atoms are discussed. Presently, GRECPs (Gatchina relativistic pseudopotentials, GRPPs) are generated for all the elements of D.I. Mendeleev Periodic Table, including heavy transition metals (d-elements), lanthanides and actinides (f-elements), superheavy elements (SHEs), as well as those “with an empty core” for light elements, see section ). The pdf-files of the PhD theses and details of GRECPs can be found in the list of QPCD papers.

We use essentially similar technology to calculate the electronic structure and properties of materials: first, we calculate the periodic structure of a perfect crystal using medium-core semilocal GRPPs for heavy atoms, then we build the “compound-tunable embedding potential” or CTEP (see below) for a chosen crystal fragment. The CTEP method describes with very high accuracy the effect of the environment on a given fragment, and, accordingly, the electronic structure of the crystal fragment itself is also reproduced properly. Finally, a two-component calculation of the crystal fragment ("cluster calculation") is performed using the CTEP embedding potential, precision versions of the relativistic pseudopotentials for heavy atoms, and fairly complete atomic basis sets. Compared to the extended cell methods, point defects (including vacancies, actinides, lanthanides and heavy transition metals) are much simpler considered in materials within the framework of cluster calculations with CTEP and with an accuracy unattainable for computational methods using the periodic boundary conditions. In the cluster case with CTEP, the computational errors can be less than 0.1 eV for valence energies; it is possible to take into account local symmetry within the fragment that is breaking the crystal one; relativistic effects (including Breit and quantum electrodynamic); electron correlation within the framework of the wave function theory; correctly consider charged fragments of crystals and those including atoms with partially occupied core shells; study the localized (nonlinear) quantum processes, etc.

**Now we are working in the following directions of development of theory and computer codes, precision studies of various many-electron systems and their properties**:

__theory and generation of RECPs__for high precision calculations of the electronic structure of molecules and materials;__non-variational and variational restoration__- quantum electrodynamics (QED) of atomic systems and those containing other elementary particles (muons/antimuons, positrons, etc.);
- compound-tanable embedding potential (CTEP) theory;
- theory of the effective state of atoms in chemical compounds;
- theory of chemical shifts of X-ray emission lines (XES);
- highly accurate relativistic methods for calculating the electronic structure of molecules and materials with heavy atoms (theory of relativistic coupled clusters in Fock space, FS-RCC); variational approaches in the quantum theory of many-electron systems);
- unitary group theory for relativistic quantum chemistry (spin-orbit interaction) and other methods;
- incorporation of GRECP methods and the Gatchina version of FS-RCC into software packages for calculating correlation structure, such as DIRAC, MOLCAS, SODCI and MRD-CI ("multi-reference configuration interaction with single and double excitations"), CI/MBPT (configuration interaction / many-particle perturbation theory);
__the EXP-T and LIBGRPP program systems__for relativistic multi-reference coupled cluster calculations of molecules and crystal fragments;- search for “new physics” (physics beyond the Standard Model, dark matter) on molecules and solids; effects of non-conservation of temporal invariance (T) and spatial parity (P);
- physics and chemistry of transition metals, lanthanides, actinides, and superheavy elements;
- localized (nonlinear) quantum processes and point defects in materials;
- magnetic structure of materials containing d- and f-elements;
- optical properties of materials (solar energy, photovoltaics, light sources, phosphors, etc.);
- calculations of physical-chemical properties of molecules and materials (including matrix elements of operators that are singular near heavy nuclei: hyperfine structure, effects of non-conservation of T-invariance and P-parity, chemical shifts of the XES and other properties);
- studies of the interaction of atoms, molecules and materials with muons and antimuons (muSR method);
- quantum electrodynamic studies of the properties of highly charged ions (energy levels, transition probabilities, cross-sections for processes of collisions of ions with elementary particles and light atoms);
- organometallic structures, functionalized endofullerenes for nuclear medicine, MRI, etc.

For details see our __recent publications and preprint__.

Also, see__ our scientific cooperation and grants__.

**The computer codes developed by our dept are available
by e-mail request.**

**If you are interested** in knowing more about our work, current
projects or you want to make some comments, please, __contact us__.
We are looking for interested graduate students, masters and bachelors to work in our department on their dissertations, theses and further research.