Browsing by Subject "Quantum Monte Carlo"

Quantum Monte Carlo (QMC) is an accurate but computationally expensive technique for simulating the electronic structure of solids, with its use as a simulation technique for modelling positron states and annihilation in solids relatively new. These simulations can support positron annihilation spectroscopy and help with defect characterisation in solids and vacancy identification by calculating the positron lifetime with increased accuracy and comparing them to experimental results. One method of reducing the computational cost of simulations whilst maintaining chemical accuracy is to employ pseudopotentials. Pseudopotentials are a method to approximate the interactions between the outer valence electrons of an atom and the inner core electrons, which are difficult to model. By replacing the core electrons of an atom with an effective potential, a level of accuracy can be maintained whilst reducing the computational cost. This work extends existing research with a new set of pseudopotentials with fewer core electrons replaced by an effective potential, leading to an increase in the number of core electrons in the simulation. With the inclusion of additional core electrons into the simulation, the corrections that need to be made to the positron lifetime may not be needed. Silicon is chosen as the element under study as the high electron count makes it difficult to model accurately for positron simulations. The suitability of these new pseudopotentials for QMC is shown by calculating the cohesive and relaxation energy with comparisons made to previously used pseudopotentials. The positron lifetime is calculated from QMC simulations and compared against experimental and theoretical values. The simulation method and challenges due to the inclusion of more core electrons are presented and discussed. The results show that these pseudopotentials are suitable for use in QMC studies, including positron lifetime studies. With the inclusion of more core electrons into the simulation a positron lifetime was calculated with similar accuracy to previous studies, without the need for corrections, proving the validity of the pseudopotentials for use in positron studies. The validation of these pseudopotentials enables future theoretical studies to better capture the annihilation characteristics in cases where core electrons are important. In achieving these results, it was found that energy minimisation rather than variance minimisation was needed for optimising the wavefunction with these pseudopotentials.