ELFT HÍRADÓ

[Szilard SZALAY <szalay.szilard AT wigner.mta.hu>]

Seminar of the
Wigner SzFI Strongly Correlated Systems "Lendület" Research Group




    Unconventional electronic structure methods for actinides

Pawel Tecmer
Institute of Physics, Nicolaus Copernicus University, Torun
(host: Örs Legeza)

Tuesday, 5 June 2018 14:00, KFKI Campus, Bldg. 1, 2nd floor, Conference Room

    Reliable modeling of electronic structures of actinide species
remains a real challenge for present day quantum chemistry. One of the
main difficulty is to reli ably correlate a large number of strongly
correlated electrons. Standard electronic structure methods are limited
to a small number of correlated orbitals (usually up to 18), which is
not sufficient for larger, more realistic actinide compounds. Therefore,
we have to refer to some alternative electronic structure methods that
use more efficient parametrizations of the wave function, but still lead
to trustworthy results. A remedy to this problem can be found in
geminal-based approaches and in the Density Matrix Renormalization Group
(DMRG) algorithm. Geminal-based approaches are a promising alternative
to model strongly correlated materials because they do not suffer from
the exponential scaling wall of conventional approaches in quantum
chemistry. Particularly interesting is the variationally Orbital
Optimized Antisymmetric Product of 1-reference orbital Geminals
(vOO-AP1roG) [1,2,3] method. The most important features of the AP1roG
ansatz are its mean-field-like scaling and that no active spaces have to
be specified. In the first part of my talk, I will assess the accuracy
of the vOO-AP1roG approach to singlet-ground-state chemistry, including
bond-breaking processes [3,4], and the 1-D Hubbard model [2]. Moreover,
the analysis of the symmetric dissociation of [UO_2]^{2+} will be
supported by the recently implemented orbital entanglement analysis [5].

    In the second part of my talk, I will discuss recent advances in
modeling complex electronic structures of actinide compounds using the
DMRG algorithm [6].

References:
[1] P. A. Limacher, P. W. Ayers, P. A. Johnson, P. Bultinck, S. De
Beardemacker, and D. Van Neck, J. Chem. Theory Comput., 9, 1394 (2013).
[2] K. Boguslawski, P. Tecmer, P. W. Ayers, P. Bultinck, S. De
Beardemacker, and D. Van Neck, Phys. Rev. B, 89, 201106(R) (2014).
[3] P. Tecmer, K. Boguslawski, P. W. Ayers, P. A. Johnson, P. A.
Limacher, M. Chan, T. Verstraelen, and P. W. Ayers, J. Phys. Chem. A,
118, 9058 (2014).
[4] P. Tecmer, K. Boguslawski, and P. W. Ayers, Phys. Chem. Chem. Phys.
17, 14427 (2015).
[5] K. Boguslawski and P. Tecmer, Int. J. Quantum Chem. 115, 1289-1295
(2015).
[6] P. Tecmer, K. Boguslawski, Ö. Legeza, and M. Reiher, Phys. Chem.
Chem. Phys. 16, 719 (2014).




    Strong correlation in actinide chemistry

Aleksandra Lachmanska
Institute of Physics, Nicolaus Copernicus University, Torun
(host: Örs Legeza)

Tuesday, 5 June 2018 14:30, KFKI Campus, Bldg. 1, 2nd floor, Conference Room

    Strongly-correlated systems such as actinide-containing molecules
constitute a challenge in present-day quantum chemistry. The
difficulties in actinide chemistry start on the atomic level where
electrons occupy nearly-degenerate 5f, 6d, and 7s orbitals and should be
described using the laws of (special) relativity. Their peculiar
behaviour leads to characteristic features and complex formations of
actinide compounds [1]. One example of such an interesting behaviour are
cation–cation interactions. The attraction between single cations leads
to the formation of T-shaped and diamond-shaped neptunyl dimers in
solution. In this talk, I will elucidate their bonding mechanism by
scrutinizing their electronic structure, which requires multi-reference
methods to be modelled accurately. Furthermore, their correlated nature
will be clarified using entropic measures arising from quantum
information theory [2].

    Neptunyl clusters are a key example for the difficulty in choosing a
proper zeroorder wave function that allows us to capture strong
correlation using traditional methods. Sophisticated computational
routines and reasonable approximations are necessary to capture the main
characteristic physical and chemical properties of actinide compounds in
calculations that are computationally feasible. This problem can be
tackled efficiently in the pair Coupled Cluster Doubles (pCCD) model
combined with variationally optimized orbitals. Specifically, it was
shown that the pCCD method captures the major part of the static
correlation in certain well studied molecules such as UO_2^{2+}.
However, the method does not account for all correlation effects [3].
One way to include the missing (dynamic) electron correlation effects is
to use a (linearized) Coupled Cluster correction on top of the pCCD wave
function [4,5]. The frozen-pair Coupled Cluster Singles Doubles (fpCCSD)
method is the alternative efficient way to improve the wave function
with the costof CCSD [4]. In fpCCSD or its linearised variant, the
singles and non-pair doubles amplitudes are optimized, while the pair
doubles amplitudes are kept frozen. Such an optimization routine
provides a balanced description of strongly-correlated systems, where
traditional CCSD usually fails. In the final part of my talk, I will
scrutinize the performance of various Coupled Cluster corrections on top
of pCCD.

References:
[1] L. R. Morss, N. Edelstein, J. Fuger, and J. J. Katz, editors. The
chemistry of the actinide and transactinide elements. Springer,
Dordrecht, 4th edition, 2011.
[2] A. Lachmańska, P. Tecmer, Ö. Legeza, and K. Boguslawski. Elucidating
cation–cation interactions in neptunyl dications using multireference ab
initio theory. under revision in Inorg. Chem.
[3] K. Boguslawski, P. Tecmer, P. W. Ayers, P. Bultinck, S. de
Baerdemacker, and D. van Neck. Efficient description of strongly
correlated electrons with mean-field cost. Phys. Rev. B, 89, 201106 (2014).
[4] Th. M. Henderson, I. W. Bulik, T. Stein, and G. E. Scuseria.
Seniority-based coupled cluster theory. J. Chem. Phys., 141, 244104 (2015).
[5] K. Boguslawski and P. W. Ayers. Linearized coupled cluster
correction on the antisymmetric product of 1-reference orbital geminals.
J. Chem. Theory Comput., 11, 5252 (2015).




Everyone is welcome to attend.

-- Szilárd Szalay. --