#### Report on the outcomes of a Short-Term Scientific Mission

**Action number:** CA21101

**Grantee name:** Dariusz Kędziera

#### Details of the STSM

**Title:** Basis set optimization for the accurate description of the optical properties of halogen containing squaraines in solution

**Start and end date:** 30/01/2022 to 10/02/2022

#### Description of the work carried out during the STSM

The internship at Dr. Heriber Reis in Athens aimed to obtain a basis set allowing for the effective determination of dispersive interaction energy for halogen bonds. The prototype systems were pyridine and CXF3, where X is F, Cl, Br, and I. The def2-tzvp and def2-qzvp basis were used as a target for modification. Initially, I planned to optimize the bases for halogens only. But during the calculations, it turned out that the basis set optimization for nitrogen can significantly improve the obtained results. Hence, I extended the project by optimizing the exponents for the nitrogen atom. In this case, the averaged E20disp value was optimized for Pyridine-CIF3, Pyridine-CBrF3, Pyridine-CClF3 and Pyridine-CFF3, calculated in their equilibrium geometries. The following activities were carried out during the internship:

1) using the SAPT method in the SAPT0 variant, optimal geometries were determined for Pyridine-CXF3 systems (rigid monomers). A strong dependence of the geometry on the used basis set was observed. For example, moving from def2-tzvp to def2-qzvpd extended by mid-bond functions, the N-I from 2.67A to 2.57A.

2) Optimization of the additional exponents s, p, d, and f for def2-tzvp against the E20 disp dispersion energy for N, F, Cl, Br, and I atoms was performed.

3) Modified basis sets were tested by determining potential energy curves for Pyridine-CXF3 systems. Def2-tzvp basis sets were extended in the series +1s, +1s1p, +1s1p1d, and +1s1p1d1f both for the X atom and for nitrogen.

4) Optimization of the additional exponents s, p, d, and f for the base def2-qzvp against the E20disp dispersion energy for N, F, Cl, Br, and I atoms was performed. However, in this case, increasing the base did not significantly improve the results. Only adding the exponent of g type gives a noticeable increase in energy. The g exponents were optimized, but due to the large size of the resulting basis set, they were not considered further.

5) Tests of the basis set with the systems from the XB51 set have been started.

#### Description of the STSM main achievements and planned follow-up activities

Extending the def2-tzvp basis set for halogens obtained during the STMS leads to results similar to those obtained using a much larger basis set like def2-qzvp, which means a significant improvement in results with relatively little computational effort. Using a modified basis set shortens the halogen bond in the Pyridine-CXF3 system and increases the interaction energy. For example, for the Pyridine-CIF3 system, in the case of the def2-tzvp basis set in the SAPT0 method, we obtain bond length 2.679A and interaction energy -9.30kcal/mol. The def2-qzvp basis for N and I yields 2.601A and -10.75kcal/mol, respectively. Enriching def2-tzvp basis set with exponents 1s1p1d on I and N leads to 2.642A and – 10.10kcal/mol. By increasing the basis by one exponent f, i.e., adding the exponents 1s1p1d1f to the original basis, we get 2.627A and -10.62kcal/mol. It is worth noting that when we subtract g exponent from def2-qzvp basis set, we get 2.629A and -10.37kcal/mol. So still more effective is to use the base def2-tzvp+1s1p1d1f (N [5s,3p,2d,1f] and I [6s,5p,3d,2f] +1s1p1d1f) than truncated base def2-qzvp (N [7s,4p,3d,2f,1g], I [7s,6p,4d,4f,1g]-1g). Tests of the basis set with the systems with halogen bonds from the XB51 set are still in progress.

The effect of the STMS, apart from determining the optimal exponents for the model system, is the development of a method for improving the functional basis for better accounting for dispersion. It is planned to automate the process by creating a program in python. The obtained basis set is used in a joint (Athens-Toruń) project devoted to the tuning of the squaraine dyes properties via the halogen bonds. It will provide the ground to develop force field parameters for MD calculations with investigated squaraines. The currently created basis set is used for finding optimal geometries of squaraine dyes complexes with CF3I and Pyridine. The results of these studies will be made public in a publication. The results obtained during the STMS were presented (poster) at the 1st COSY General Meeting in Cadiz.