We are interested/working in the following areas

**Tensor network methods for strongly correlated quantum chemistry**

Tensor network methods represent

*efficient* wave function parametrizations especially suitable for

*strongly correlated* (multireference) problems. Even the simplest, so called matrix product states (MPSs), which are optimized by the density matrix renormalization group (DMRG) method, proved themselves as very robust and reliable tools. In fact, DMRG has been established as a reference method for electronic properties of systems requiring large active spaces.

Recenlty, in collaboration with

Jiří Brabec (Heyrovský Institute),

Karol Kowalski (PNNL, USA), and

Örs Legeza (Wigner Research Center, Budapest), we have developed the

*massively parallel* version of DMRG, which can treat even larger active spaces and thus more complex systems.

One of our further aims is to generalize this implementation for tree tensor network topologies.

*Selected publications:*

Jiří Brabec,

**Jan Brandejs**, Karol Kowalski, Sotiris Xantheas, Örs Legeza,

**Libor Veis**,

*Massively parallel quantum chemical density matrix renormalization group method*,

arXiv:2001.04890.

**Applications of tensor network methods (DMRG in particular) on challenging strongly correlated problems**

We do not only develop computational methods, but also apply them on interesting and challenging problems. Typical systems studied by DMRG are transition metal complexes. In collaboration with

Pavel Hobza's group at the Institute of Organic Chemistry and Biochemistry, we have recently studied the properties of low lying spin states of porphyrin-like molecules.

Other examples include multi-radical polycycylic aromatic hydrocarbons derivatives stabilized by metal surfaces, in collaboration with

Pavel Jelínek's group at the Institute of Physics.

*Selected publications:*

Andrej Antalík, Dana Nachtigallová, Rabindranath Lo,

**Mikuláš Matoušek**, Jakub Lang, Örs Legeza, Jiří Pittner, Pavel Hobza,

**Libor Veis**,

*Ground State of the Fe(II)-porphyrin Model System Corresponds to the Quintet State: A DFT and DMRG-based Tailored CC Study*,

arXiv:2001.04903.

Dana Nachtigallová, Andrej Antalík, Rabindranath Lo, Robert Sedlák, Debashree Manna, Jiří Tuček, Juri Ugolotti,

**Libor ****Veis**, Örs Legeza, Jiří Pittner, Radek

Zbořil, Pavel Hobza, *Chem. Eur. J.*, **24**, 13413, (2018). DOI: 10.1002/chem.201803380

**Post-DMRG quantum chemical methods**

DMRG itself is a very powerful multireference method, which can work with large active spaces. However, it lacks the dynamical electron correlation and its inclusion is important to achieve chemical accuracy.

Recently, in collaboration with

Jiří Pittner's group at the Heyrovský Institute, we have developed the DMRG-based tailored coupled clusters (CC), in which CC in some sense adds the missing dynamical correlation on top of the DMRG wave function.

In collaboration with

Katarzyna Pernal (Lodz University of Technology), we are currently pursuing the addiabatic connection (AC) technique to add the missing dynamical correlation in the DMRG active space. The AC methods achieve the accuracy similar to the second order perturbation theory, but have favourable scaling with the active space size.

*Selected publications:*

**Libor ****Veis**, Andrej Antalík, Jiří Brabec, Frank Neese, Örs Legeza, Jiří Pittner,

*J. Phys. Chem. Lett.*,

**7**, 4072, (2016).

DOI: 10.1021/acs.jpclett.6b01908
Ewa Pastorczak, Ewa Hapka, Michal Hapka,

**Libor**** ****Veis**, Katarzyna Pernal,

*J. Phys. Chem. Lett.*,

**10**, 4668, (2019).

DOI: 10.1021/acs.jpclett.9b01582

**Quantum computing for quantum chemistry**

Quantum computers have a potential to change the way we are doing chemistry!

The idea of Feynman to simulate one quantum system on another and thus to avoid the complexity problems associated with simulations of quantum systems on a classical computer were 15 years ago brought also to quantum chemistry. Nowdays, quantum computing for quantum chemistry is a well-established and fast-growing discipline of theoretical chemistry.

Though it may take a while when large-enough universal quantum computers are available, even the near-term "few"-qubit devices might offer great opportunities for doing things that are classically impossible.

*Selected publications:*

Yudong Cao, Jonathan Romero, Jonathan P. Olson, Matthias Degroote, Peter D. Johnson, Maria Kieferova, Ian D. Kivlichan, Tim Menke, Borja Peropadre, Nicolas P. D. Sawaya, Sukin

Sim, **Libor ****Veis**, Alán Aspuru-Guzik, *Chem. Rev.*, **119**, 10856, (2019). DOI: 10.1021/acs.chemrev.8b00803

**Libor Veis**, Jiří Pittner, *J. Chem. Phys.*, **113**, 194106, (2010). DOI: 10.1063/1.3503767