One of the central pursuits of modern theoretical chemistry and condensed matter physics has been to elucidate and explain the often complex array of microscopic interactions that underpin properties of molecular and material phase diagrams. Despite decades of intensive research, however, much remains to be understood about phase transitions in many materials, very prominently including d- and f-electron-containing materials such as transition metal oxides, lanthanides, and actinides. First-principle calculations of materials at finite temperatures remain a formidable challenge, in large part because these not only requires being able to capture thermal effects, but also being able to correctly handle electron correlation and the large eigenvalue spread that accompanies realistic Hamiltonians. In the present work, we extend our previous research developing a finite temperature Auxiliary Field Quantum Monte Carlo (AFQMC) method capable of treating arbitrary ab initio Hamiltonians to the treatment of realistic solids. We first adapted the finite temperature AFQMC formalism to model ab initio Hamiltonians and establish a set of energy benchmarks for molecules at finite temperature where the phase problem is well-controlled [1]. Here, we further adapt this formalism to the more natural setting of solids and use it to study Mott insulator transitions in equidistance H10 chains with long-range Coulomb interactions. Convergence with respect to Brillouin zone sampling is achieved at various H-H bond lengths. The internal energy, heat capacity, and double occupancy at finite temperatures clearly suggest a gradual transition of the system from metal to Mott insulator as the H-H distance is increased. The transition is sharper at lower temperatures, indicating that quantum fluctuations dominate the transition. Our work provides a powerful numerical tool to study the finite temperature phase diagrams of realistic solids and strongly correlated systems.

Probing metal-insulator phase transitions in realistic solids via ab initio finite temperature auxiliary field quantum Monte Carlo

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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