We present g-xTB, a next-generation semi-empirical electronic structure method derived from tight-binding (TB) approximations to Kohn–Sham density functional theory (KS-DFT). Designed to bridge the gap between semi-empirical quantum mechanical (SQM) approaches and DFT in terms of accuracy, robustness, and general applicability, g-xTB targets the performance of the ωB97M-V range-separated hybrid density functional with large basis sets while maintaining TB speed. Key innovations include an atom-in-molecule adaptive atomic orbital basis, a refined Hamiltonian incorporating range-separated approximate Fock exchange, up to fourth-order charge-fluctuation terms with a novel first-order electronic contribution, and atomic correction potentials (ACPs), as well as a charge-dependent semi-classical repulsion function. Parameterized on extended and extremely diverse molecular training sets – including “mindless molecules” – g-xTB achieves excellent accuracy across a broad chemical space, including the actinide elements. Benchmarking against around 32k relative energies across thermochemistry, conformational energetics, non-covalent interactions, and reaction barriers shows that g-xTB consistently outperforms GFN2-xTB, often reducing mean absolute errors by half. Notably, it achieves a WTMAD-2 of 9.3 kcal mol−1 on the full GMTKN55 benchmark, comparable to low-cost DFT methods. It also shows substantial improvements for transition-metal complexes, relative spin state energies, and orbital energy gaps – areas where many SQM and even DFT methods often struggle. In summary, g-xTB offers DFT-like accuracy with minimal computational overhead compared to its predecessor, GFN2-xTB, making it a robust, minimally empirical, transferable, and efficient alternative to machine learning interatomic potentials for a wide range of molecular simulations. It is proposed as a general replacement for the GFNn-xTB family and, in many practical cases, a viable substitute for low- and mid-level DFT methods.

We present g-xTB, a next-generation semi-empirical electronic structure method derived from tight-binding (TB) approximations to Kohn–Sham density functional theory (KS-DFT). Designed to bridge the ga...

Electronegativity equilibration model for atomic partial charges - GitHub - thfroitzheim/multicharge at eeq-bc

The accurate and efficient assignment of atomic partial charges is crucial for many applications in theoretical and computational chemistry, including polarizable force fields, dispersion corrections, and charge-dependent basis sets. Classical charge models struggle to distinguish between neutral and zwitterionic fragments because, unlike quantum mechanical methods, there are no discrete electronic states. This limitation can lead to either reduced or additional artificial charge transfer (CT) at different interfragment distances. To address this issue, we propose a new version of a bond capacity electronegativity equilibration (EEQBC) model, which limits artificial CT between distant fragments in the simple EEQ framework. EEQBC offers excellent agreement with DFT-based reference charges for elements up to lawrencium (Z = 103) with mean absolute errors as low as 0.02 and 0.07 e− for random PubChem molecules and "mindless" molecules (MLMs), respectively. Thanks to its computational efficiency for both atomic charges and their analytical nuclear gradients, EEQBC is highly suitable as an initial charge guess for next-generation tight-binding methods. For seamless accessibility, EEQBC is implemented in the freely available multicharge program at: github.com/thfroitzheim/multicharge/tree/eeq-bc.

The accurate and efficient assignment of atomic partial charges is crucial for many applications in theoretical and computational chemistry, including polarizable force fields, dispersion corrections, a...

The Charge Extended Hückel (CEH) model, initially introduced for adaptive atomic orbital (AO) basis set construction (J. Chem. Phys. 2023, 159, 164108), has been significantly revised to enhance accur...