Absolute proton hydration free energy, surface potential of water, and redox potential of the hydrogen electrode from first principles: QM/MM MD free-energy simulations of sodium and potassium hydration

摘要: The absolute intrinsic hydration free energy GH+,wat◦ of the proton, surface electric potential jump χwat◦ upon entering bulk water, and redox VH+,wat◦ reference hydrogen electrode are cornerstone quantities for formulating single-ion thermodynamics on scales. They can be easily calculated from each other but remain fundamentally elusive, i.e., they cannot determined experimentally without invoking some extra-thermodynamic assumption (ETA). Born model provides a natural framework to formulate such an (Born ETA), as it automatically factors out contribution crossing water energy. However, this describes short-range solvation inaccurately relies choice arbitrary ion-size parameters. In present study, both shortcomings alleviated by performing first-principle calculations energies sodium (Na+) potassium (K+) ions. rely thermodynamic integration based quantum-mechanical molecular-mechanical (QM/MM) molecular dynamics (MD) simulations involving ion 2000 molecules. its first shell described using correlated ab initio method, namely resolution-of-identity second-order Moller-Plesset perturbation (RIMP2). next shells extended simple point charge (SPC/E). is at MM level subsequently increased quantization term accounting transformation QM/MM description. It also corrected finite-size, approximate-electrostatics, potential-summation errors, well standard-state definition. These computationally intensive provide accurate estimates GH+,wat◦, χwat◦, VH+,wat◦, reported with statistical errors confidence interval 99%. values obtained independent Na+ K+ in excellent agreement. particular, difference between two energies, which not elusive quantity, 73.9 ± 5.4 kJ mol-1 (K+ minus Na+), compared experimental value 71.7 2.8 mol-1. (-1096.7 6.1 mol-1, 0.10 V, 4.32 0.06 respectively, averaging over ions) remarkable agreement recommended Reif Hunenberger thorough analysis literature (-1100 5 0.13 4.28 respectively). MD shown description structure, dynamics, energetics.