Benchmarking semiempirical quantum chemical methods on liquid water

Stimulated by the renewed interest and recent developments in semiempirical quantum chemical (SQC) methods for noncovalent interactions, we examine the properties of liquid water under ambient conditions by means of molecular dynamics (MD) simulations, both with the conventional neglect of diatomic differential overlap-type methods, e.g., AM1 and PM6, and with DFTB-type (density-functional tight-binding) methods, e.g., DFTB2 and GFN-xTB (Geometry-Frequency-Noncovalent eXtended Tight-Binding). Besides the original parameter sets, some specifically reparameterized SQC methods (denoted as AM1-W, PM6-fm, and DFTB2-iBi) targeting various smaller water systems ranging from molecular clusters to bulk are considered as well. The quality of these different SQC methods for describing liquid water properties under ambient conditions is assessed by comparison with well-established experimental data and also with BLYP-D3 density functional theory-based ab initio MD simulations. Our analyses reveal that static and dynamic properties of bulk water are poorly described by all considered SQC methods with the original parameters, regardless of the underlying theoretical models, with most of the methods suffering from too weak hydrogen bonds and hence predicting a far too fluid water with highly distorted hydrogen bond kinetics. Meanwhile, the reparameterized force-matched PM6-fm method is shown to be able to quantitatively reproduce the static and dynamic features of liquid water and thus can be used as a computationally efficient alternative to electronic structure-based MD simulations for liquid water that requires extended length and time scales. DFTB2-iBi predicts a slightly overstructured water with reduced fluidity, whereas AM1-W gives an amorphous ice-like structure for water under ambient conditions.