Ab initio calculations of electric field gradients in H-bond rich molecular crystals with nearly experimental accuracy.

Ab initio calculations of electric field gradients (EFGs) in molecular crystals have advanced significantly due to the gauge including projector augmented wave (GIPAW) formalism, which accounts for the infinite periodicity in crystals. However, theoretical accuracies still lag behind experimental ones, making it challenging to distinguish experimentally distinguishable similar structures, a deficiency largely attributed to the limitation of GIPAW codes to generalized gradient approximation (GGA) density functional theory (DFT) functionals. In this study, we investigate whether hybrid DFT functionals can enhance the EFG calculation accuracy and the associated geometry optimization. Using the many-body expansion method, we focus on nitrogen EFGs in amino acids with complex H-bonding, which are often poorly described with GGA functionals. Our results show that both functionals provide highly accurate calculations that surpass current studies and approach experimental precision. The accuracies are also almost three times higher than available GIPAW/GGA calculations in the literature. However, we show that this difference is not due to the GGA functional but rather due to the improper selection of the nitrogen quadrupole moment.