Quantum alchemy beyond singlets: Bonding in diatomic molecules with hydrogen

Bonding energies are key for the relative stability of molecules in chemical space. Therefore methods employed to search for relevant molecules in chemical space need to capture the bonding behavior for a wide range of molecules, including radicals. In this work, we investigate the ability of quantum alchemy to do so for exploring hypothetical chemical compounds, here diatomic molecules involving hydrogen with various electronic structures. We evaluate equilibrium bond lengths, ionization ener- gies, and electron affinities of these fundamental systems. We compare and contrast how well manual quantum alchemy calculations, i.e. quantum mechanical calculations in which the nuclear charge is altered, and quantum alchemy approximations using a Taylor series expansion can predict these molecular properties. We also investigate the extent of error cancellation of these approaches in terms of ionization energies and electron affinities when using thermodynamic cycles. Our results suggest that the accuracy of Taylor series expansions are greatly improved by error cancellation in thermodynamic cycles, and errors also appear to be generally system-dependent. Taken together, this work provides insights into how quantum alchemy predictions us- ing a Taylor series expansion may be applied to future studies of non-singlet systems as well as which challenges remain open for these cases.

Two Faces of the Two Phase Thermodynamic Model

<div>The quantum harmonic model and the two-phase thermodynamics method (2PT) are widely used to obtain quantum corrected properties such as isobaric heat capacities or molar entropies. 2PT heat capacities were calculated inconsistently in the literature, and the excellent correlations are due to error cancellation for organic liquids. We reanalyzed the performance of different quantum corrections on the heat capacities of common organic solvents against experimental data. The accuracy of the computations was also assessed with the determination of the self-diffusion coefficients.</div><div><br></div>

Two Faces of the Two Phase Thermodynamic Model

<div>The quantum harmonic model and the two-phase thermodynamics method (2PT) are widely used to obtain quantum corrected properties such as isobaric heat capacities or molar entropies. 2PT heat capacities were calculated inconsistently in the literature, and the excellent correlations are due to error cancellation for organic liquids. We reanalyzed the performance of different quantum corrections on the heat capacities of common organic solvents against experimental data. The accuracy of the computations was also assessed with the determination of the self-diffusion coefficients.</div><div><br></div>