Reduced Density Matrix Formulation of Quantum Linear Response.

The prediction of spectral properties via linear response (LR) theory is an important tool in quantum chemistry for understanding photoinduced processes in molecular systems. With the advances of quantum computing, we recently adapted this method for near-term quantum hardware using a truncated active space approximation with orbital rotation, named quantum linear response (qLR). In an effort to reduce the classic cost of this hybrid approach, we here derive and implement a reduced density matrix (RDM) driven approach of qLR. This allows for the calculation of spectral properties of moderately sized molecules with much larger basis sets than so far possible. We report qLR results for benzene and R-methyloxirane with a cc-pVTZ basis set and study the effect of shot noise on the valence and oxygen K-edge absorption spectra of H2O in the cc-pVTZ basis.

A variational reformulation of molecular properties in electronic-structure theory.

Conventional quantum-mechanical calculations of molecular properties, such as dipole moments and electronic excitation energies, give errors that depend linearly on the error in the wave function. An exception is the electronic energy, whose error depends quadratically on the error in wave function. We here describe how all properties may be calculated with a quadratic error, by setting up a variational Lagrangian for the property of interest. Because the construction of the Lagrangian is less expensive than the calculation of the wave function, this approach substantially improves the accuracy of quantum-chemical calculations without increasing cost. As illustrated for excitation energies, this approach enables the accurate calculation of molecular properties for larger systems, with a short time-to-solution and in a manner well suited for modern computer architectures.

The effects of solvation on the back reaction and storage capabilities of solar thermal energy storage systems.

Solvent effects on molecular solar thermal energy storage systems have been investigated using density functional theory combined with solvent models describing the effects of viscosities and dielectric constants on chemical reaction rates. We have addressed the following issues concerning how solvents influence both the thermochemical properties and the thermal relaxation kinetics of the studied systems, how the friction of the solvent influences the recrossing of the reactions along with the dynamics and force constants of the transition state. We observe that the rate constants for the chemical reactions of the molecular solar thermal energy storage systems depend strongly on the dielectric solvent properties and the viscosities of the solvents.