The goal of the project is to develop methods for reliable calculation of binding energies of molecular solids.
To reach this goal we worked on three main topics: i) accuracy, ii) precision, and iii) reference data.
In the first part, we study accuracy of methods that are used to calculate binding energies or similar properties
for molecular solids or clusters. We focus on density functional theory (DFT) approximations and random
phase approximation (RPA). As a reference, we use a coupled clusters scheme CCSD(T).
To understand the accuracy for molecular systems we divide them into smaller fragments, such as molecular
dimers, trimers, or tetramers. For the clusters we calculate binding energies with the reference method
and with the tested schemes. We then calculate the errors of the simpler methods and analyze how they
depend on different parameters or how they behave for different systems. Within the project we performed
this analysis for methane clathrate cluster (methane molecule in a cage of twenty water molecules), for crystals
of short hydrocarbons, and for other systems. We found that the simple DFT functionals lead to substantial errors,
especially for errors in clusters with three or four molecules. This is quite a problem as there is no simple and
computationally cheap way to correct for these errors. We also found that the errors are largely inherited in
RPA calculations which are based on the DFT input. We found that the errors are reduced when Hartree-Fock
exchange is included, even though this can lead to substantial increase of computational cost. The work was
published in three articles so far with two more articles close to being submitted. Furthermore, we used the results
and understanding to publish other studies and also to take part in a test set of methods for predicting polymorphism,
which will lead to other publication. Moreover, we will use the developed methods to in a COST project focused
on polymorphism and in further studies of properties of molecular solids.
Concerning precision we assessed the effect of different approximations on the results. For example, we compared
binding energies calculated within periodic boundary conditions to those obtained from the smaller fragments.
We were able to obtain a close agreement between both approaches and discovered several unexpected problems
that can deteriorate the precision of the result. We also found several ways how to substantially speed up the calculations
without affecting the precision. In another work we analyzed how approximate treatment of core electron affects
the binding energies. We were able to identify the cause and develop a correction for this error. This code to
calculate the correction is available and easy to be implemented in computational packages. We hope that it will
be used to reduce the errors which can much larger than the errors of theoretical methods.
Finally, we have generated a large amount of data and we made them available, together with various scripts
for their set-up and analysis. The reference data can be used by other researchers to test their methods and should
be helpful in developing improved methods.
