  Metal oxide nanoparticles (NPs) are regarded as good candidates for many
technological applications, where their functional environment is often an
aqueous solution. The correct description of metal oxide electronic structure
is still a challenge for local and semilocal density functionals, whereas
hybrid functional methods provide an improved description, and local atomic
function based codes such as CRYSTAL17 outperform plane wave codes when it
comes to hybrid functional calculations. However, the computational cost of
hybrids are still prohibitive for systems of real sizes, in a real environment.
Therefore, we here present and critically assess the accuracy of our
electrostatic embedding quantum mechanical/molecular mechanical (QM/MM)
coupling between CRYSTAL17 and AMBER16, and demonstrate some of its
capabilities via the case study of TiO2 NPs in water. First, we produced new
Lennard-Jones (LJ) parameters that improve the accuracy of water-water
interactions in the B3LYP/TIP3P coupling. Then, we applied our QM/MM coupling
methodology to describe the interaction of a 1 nm wide multilayer of water
surrounding a spherical TiO2 nanoparticle (NP). Optimizing the QM/MM water
water parameters was found to have little to no effect on the local NP
properties, which provide insights into the range of influence that can be
attributed to the LJ term in the QM/MM coupling. The effect of adding
additional water in an MM fashion on the geometry optimized nanoparticle
structure is small, but more evident effects are seen in its electronic
properties. We also show that there is good transferability of existing QM/MM
LJ parameters for organic molecules water interactions to our QM/MM
implementation, even though these parameters were obtained with a different QM
code and QM/MM implementation, but with the same functional.
