1:a NT-Forskningskonferensen Karlstad 5 maj 2015
ON THE USEFULNESS OF COMPUTATIONAL
CHEMISTRY
K. Andersson
Institutionen för Matematik och Datavetenskap.
Tel.:054-7001873, E-mail: kerstin.andersson@kau.se
Abstract: Computational chemistry and physics can be considered as bridging the gap
between theory and experiment. Computations can give detailed explanations of processes
that are not so easily captured in experiments. In the presentation an introduction of
computational chemistry will be given together with some example problems, where
computational chemistry can be useful for the research conducted at Karlstad University.
INTRODUCTION
Computational chemistry attempts to calculate, using
computers, the predictions of quantum theory for
molecular systems and solids. Central to the calculations
is the many-body problem, which usually requires
enormous computer resources for accurate solutions. By
various approximations (and/or the inclusion of
experimental data) the application domain can be
enlarged. In the presentation only accurate ab initio
methods will be considered and therefore the application
domain necessarily is restricted to fairly small model
systems.
The calculations have been performed using the
MOLCAS quantum chemistry software package
(www.molcas.org). The central ingredient in this
package is the multi-configurational wave function (in
the complete active space (CAS) self-consistent field
(SCF) framework). This allows the calculation of
qualitatively correct wave functions (beyond the HartreeFock level) for both ground and excited electronic states.
By incorporating dynamical electron correlation effects
through second-order perturbation theory (in energy)
also quantitatively correct descriptions are given,
important for accurate calculations of energy differences.
In the example problems discussed below the calculation
of energy differences is a common factor.
EXAMPLE MODEL SYSTEMS
In order to give a flavor of the kind of calculations that
can be performed, two problem areas will be covered,
both present in the research conducted at Karlstad
University. Firstly, a solid-state problem (steel) will be
discussed and secondly an organic chemical problem
(organic solar cells) will be considered.
Steel
In order to understand the behavior of steel alloys,
molecular dynamics (MD) simulations can be performed
(Holleboom and Alasqalani, 2015). The forces between
particles can be estimated through pair potentials. The
pair potential between one pair of atoms in steel
(vanadium and carbon) has been calculated using the
MOLCAS software (see Fig. 1). The dissociation energy
(an energy difference) of 0.170 H obtained using secondorder perturbation theory (CASPT2) compares well with
the experimental value of 0.160 ± 0.009 H (Gupta and
Gingerich, 1981). However, even though the pair
potential can be considered accurate it is still an
approximation in MD simulations.
Fig. 1. Pair potential for vanadium and carbon
Organic solar cells
The active layer of an organic solar cell typically
consists of a mixture of electron donor molecules
(polymers), electron acceptor molecules (polymers or
fullerene derivatives) and solvent molecules. By an
applied electric field a direction is enforced on the solar
cell bulk material which drives the photo-excited
electrons and their corresponding holes (see step 1 in
Fig. 2) to different electrodes and thereby generates a
photocurrent. The holes are transported along the donor
polymer chain and the electrons by hopping between
acceptor fullerene molecules. The central process in the
working of a solar cell is the formation of a charge
transfer (CT) complex at the interface between donor
and acceptor material (see step 3 in Fig. 2)l. A proper
understanding of the mechanisms around the CT
complex, which involve non-equilibrium conditions,
excited states, recombination (radiative/non-radiative)
etc., may be achieved using computational chemistry.
Here it is essential that energy differences are properly
described since it is the relative positions of energy
levels that are critical (see the black bars in Fig. 2).
As a concluding remark the following quotation from the
pedagogical and visionary article in Science by Sariciftci
et al. (1992) is given:
“The wide range of possible conducting polymers and
conjugated oligomers available as donors and C60
derivatives (for example in polymeric form) as acceptors
implies an important scientific opportunity.”
ACKNOWLEDGEMENTS
The author is grateful to L. J. Holleboom for generating
the graphs in Fig. 1 and to A. Alasqalani for providing a
nice problem (VC).
REFERENCES
Gupta S. K. and K. A. Gingerich (1981), Mass
spectrometric study of the stabilities of gaseous
carbides of vanadium, niobium, and molybdenum, J.
Chem. Phys., Vol. 74, pp. 3584-3590.
Holleboom L. J. and A. Alasqalani (2015), Oral
communication.
Ma, Z. (2013), Studies of morphology and chargetransfer in bulk-heterojunction polymer solar cells,
LiU-Tryck, Linköping, Sweden.
Sariciftci N. S., L. Smilowitz, A. J. Heeger and F. Wudl
(1992), Photoinduced electron transfer from a
conducting polymer to buckminsterfullerene,
Science, Vol. 258, pp. 1474-1476.
Fig. 2. The working principle of a solar cell (Ma, 2013)
According to Sariciftci et al. (1992) the electron transfer
(step 3 in Fig. 2) describes the formation of an ion
radical pair and they give criteria for this to occur, which
involves ionization potentials, electron affinities and
electronic energies of ground and excited states of the
participating donor and acceptor molecules. Again
energy differences are of importance and it is critical to
choose quantum chemical methods that describe these
well. In some preliminary studies, using the MOLCAS
software, the lowest electronic states (both ground and
excited), ionization potentials and electron affinities
have been calculated for some simple conjugated cycle
compounds. All these properties could be calculated with
an error compared to experiment of less than 0.2 eV for
benzene (C6H6) and indenyl (C9H7). In the presentation
more results and details of the calculations will be given.
These initial studies will continue by including more and
larger conjugated cycle compounds (models of acceptor
molecules) and linear conjugated compounds (models of
donor molecules). Further, by combining a linear
conjugated compound and a cyclic conjugated
compound, charge-transfer states will be calculated. The
gole is, by this systematic approach, to find patterns and
rules that may be useful in the design of organic solar
cells.