Quantum chemical methods are computationally
expensive, especially for large molecules. Therefore, many approximate
reduced-scaling methods are proposed. In this work elongation (ELG), one of
such methods is considered. ELG is based on simulation of the polymerization
process. In this method, a polymer is divided into small units. On every step
of calculation molecular orbitals of only several units are calculated, and
then they are localized. On the next step next several units are calculated and
so on, until the last unit is reached. Reduced-scaling is reached since only a small
part of the polymer is considered along with the whole calculation. In previous
works, electrostatic embedding was implemented via the introduction of partial
atomic charges on positions of atoms in units not included in the current step
of the calculation. This approach improved the reproduction of electronic
structure of full polymer via the inclusion of long-range electrostatic interactions.
In this work, we introduce also mechanical embedding in ELG based on ONIOM
method. Following the ONIOM, all system should be divided into parts simulated
on the different level of theory – high-level and low-level parts. In this
approach, we consider simulated units as high-level part and all other units as
low-level part. Therefore, the high-level part is conventional ELG, and
low-level part provides mechanical constraints for units in high-level part. Mechanical
embedding combined with electrostatic embedding inside conventional ELG is
called elongation with mechanical and electrostatic embedding (ELG-IMEE). Such an
approach improves geometry optimization of the full polymer. Results of
optimization for a number of polymers will be shown in the presentation.