Resonating Valence Bond Quantum Monte Carlo: Application to the ozone molecule

We study the potential energy surface of the ozone molecule by means of Quantum Monte Carlo simulations based on the resonating valence bond concept. The trial wave function consists of an antisymmetrized geminal power arranged in a single-determinant that is multiplied by a Jastrow correlation factor. Whereas the determinantal part incorporates static correlation effects, the augmented real-space correlation factor accounts for the dynamics electron correlation. The accuracy of this approach is demonstrated by computing the potential energy surface for the ozone molecule in three vibrational states: symmetric, asymmetric and scissoring. We find that the employed wave function provides a detailed description of rather strongly-correlated multi-reference systems, which is in quantitative agreement with experiment.Comment: 5 page, 3 figure

Quantum Monte Carlo Study of High Pressure Solid Molecular Hydrogen

We use the diffusion quantum Monte Carlo (DMC) method to calculate the ground state phase diagram of solid molecular hydrogen and examine the stability of the most important insulating phases relative to metallic crystalline molecular hydrogen. We develop a new method to account for finite-size errors by combining the use of twist-averaged boundary conditions with corrections obtained using the Kwee-Zhang-Krakauer (KZK) functional in density functional theory. To study band-gap closure and find the metallization pressure, we perform accurate quasi-particle many-body calculations using the $GW$ method. In the static approximation, our DMC simulations indicate a transition from the insulating Cmca-12 structure to the metallic Cmca structure at around 375 GPa. The $GW$ band gap of Cmca-12 closes at roughly the same pressure. In the dynamic DMC phase diagram, which includes the effects of zero-point energy, the Cmca-12 structure remains stable up to 430 GPa, well above the pressure at which the $GW$ band gap closes. Our results predict that the semimetallic state observed experimentally at around 360 GPa [Phys. Rev. Lett. {\bf 108}, 146402 (2012)] may correspond to the Cmca-12 structure near the pressure at which the band gap closes. The dynamic DMC phase diagram indicates that the hexagonal close packed $P6_3/m$ structure, which has the largest band gap of the insulating structures considered, is stable up to 220 GPa. This is consistent with recent X-ray data taken at pressures up to 183 GPa [Phys. Rev. B {\bf 82}, 060101(R) (2010)], which also reported a hexagonal close packed arrangement of hydrogen molecules

Unconventional phase III of high-pressure solid hydrogen

We reassess the phase diagram of high-pressure solid hydrogen using mean-field and many-body wave function based approaches to determine the nature of phase III of solid hydrogen. To discover the best candidates for phase III, density functional theory calculations within the meta-generalized gradient approximation by means of the strongly constrained and appropriately normed (SCAN) semilocal density functional are employed. We study eleven molecular structures with different symmetries, which are the most competitive phases, within the pressure range of 100 to 500~GPa. The SCAN phase diagram predicts that the $C2/c-24$ and $P6_122-36$ structures are the best candidates for phase III with an energy difference of less than 1~meV/atom. To verify the stability of the competitive insulator structures of $C2/c-24$ and $P6_122-36$, we apply the diffusion Monte Carlo (DMC) method to optimise the percentage $\alpha$ of exact-exchange in the trial many-body wave function. We found that the optimised $\alpha$ equals to $40 \%$, and denote the corresponding exchange and correlation functional as PBE1. The energy gain with respect to the well-known hybrid functional PBE0, where $\alpha = 25\%$, varies with density and structure. The PBE1-DMC enthalpy-pressure phase diagram predicts that the $P6_122-36$ structure is stable up to 210~GPa, where it transforms to the $C2/c-24$. Hence, we predict that the phase III of high-pressure solid hydrogen is polymorphic.Comment: Accepted for publication in Phys. Rev.

Correlation energy of the spin-polarized electron liquid by quantum Monte Carlo

Variational and diffusion quantum Monte Carlo (VMC and DMC) methods with Slater-Jastrow-backflow trial wave functions are used to study the spin-polarized three-dimensional uniform electron fluid. We report ground state VMC and DMC energies in the density range $0.5 \leq r_\text{s} \leq 20$. Finite-size errors are corrected using canonical-ensemble twist-averaged boundary conditions and extrapolation of the twist-averaged energy per particle calculated at three system sizes (N=113, 259, and 387) to the thermodynamic limit of infinite system size. The DMC energies in the thermodynamic limit are used to parameterize a local spin density approximation correlation function for inhomogeneous electron systems.Comment: arXiv admin note: substantial text overlap with arXiv:2209.1022

Correlation energy of the paramagnetic electron gas at the thermodynamic limit

The variational and diffusion quantum Monte Carlo methods are used to calculate the correlation energy of the paramagnetic three-dimensional homogeneous electron gas at intermediate to high density. Ground state energies in finite cells are determined using Slater-Jastrow-backflow trial wave functions, and finite-size errors are removed using twist-averaged boundary conditions and extrapolation of the energy per particle to the thermodynamic limit of infinite system size. Our correlation energies in the thermodynamic limit are lower (i.e., more negative, and therefore more accurate according to the variational principle) than previous results, and can be used for the parameterization of density functionals to be applied to high-density systems

Improving Postoperative Pain Control After Cesarean Delivery with Enhanced Recovery in Patients on Buprenorphine Therapy

Abstract: Introduction: Prescription drug abuse presents a significant challenge to the management of post operative pain.Pain control amongst the opioid addicted patient can be especially challenging. We aimed to improve pain control after caesarian delivery with enhanced recovery in patients who are on buprenorphine medication-assisted therapy for the treatment of opioid addiction. Materials and Methods: We conducted a pilot study by implementing a protocol using liposomal bupivacaine injected at the time of cesarean delivery. Patients were then given 500mg acetaminophen every 4 hours, 800mg ibuprofen every 8 hours and 0.3mg IV buprenorphine every 6 hours as needed. The patients’ maintenance dosing of buprenorphine was divided into doses throughout the day. In addition, patients were ambulated 4 hours after surgery and had their catheter removed from their bladder as soon as they could safely ambulate. Eleven patients were prospectively recruited and then compared to a retrospective sample of seventeen patients. Results: Patients in the treatment group reported 27% lower pain scores (p Discussion: Our enhanced recovery protocol is an effective alternative to traditional pain control and is associated with a significant reduction in both pain scores and use of breakthrough IV buprenorphine as well as lower charges