Non-perturbative many-body treatment of molecular magnets.

Molecular magnets have received significant attention because of their potential applications in quantum information and quantum computing. A delicate balance of electron correlation, spin-orbit coupling (SOC), ligand field splitting, and other effects produces a persistent magnetic moment within each molecular magnet unit. The discovery and design of molecular magnets with improved functionalities would be greatly aided by accurate computations.

Ab initio calculations in atoms, molecules, and solids, treating spin-orbit coupling and electron interaction on an equal footing.

We incorporate explicit, non-perturbative treatment of spin-orbit coupling into ab initio auxiliary-field quantum Monte Carlo (AFQMC) calculations. The approach allows a general computational framework for molecular and bulk systems in which material specificity, electron correlation, and spin-orbit coupling effects can be captured accurately and on an equal footing, with favorable computational scaling vs system size. We adopt relativistic effective-core potentials that have been obtained by fitting to fully relativistic data and that have demonstrated a high degree of reliability and transferability in molecular systems.

Local Embedding and Effective Downfolding in the Auxiliary-Field Quantum Monte Carlo Method.

A local embedding and effective downfolding scheme has been developed and implemented in the auxiliary-field quantum Monte Carlo (AFQMC) method. A local cluster in which electrons are fully correlated is defined, and the frozen orbital method is used on the remainder of the system to construct an effective Hamiltonian, which operates within the local cluster. Local embedding, which involves only the occupied sector, has previously been employed in the context of Co/graphene.

Auxiliary-field quantum Monte Carlo calculations of the molybdenum dimer.

Chemical accuracy is difficult to achieve for systems with transition metal atoms. Third row transition metal atoms are particularly challenging due to strong electron-electron correlation in localized d-orbitals. The Cr2 molecule is an outstanding example, which we previously treated with highly accurate auxiliary-field quantum Monte Carlo (AFQMC) calculations [W.

Quantum Monte Carlo Calculations in Solids with Downfolded Hamiltonians.

We present a combination of a downfolding many-body approach with auxiliary-field quantum Monte Carlo (AFQMC) calculations for extended systems. Many-body calculations operate on a simpler Hamiltonian which retains material-specific properties. The Hamiltonian is systematically improvable and allows one to dial, in principle, between the simplest model and the original Hamiltonian.

An auxiliary-field quantum Monte Carlo study of the chromium dimer.

The chromium dimer (Cr2) presents an outstanding challenge for many-body electronic structure methods. Its complicated nature of binding, with a formal sextuple bond and an unusual potential energy curve (PEC), is emblematic of the competing tendencies and delicate balance found in many strongly correlated materials. We present an accurate calculation of the PEC and ground state properties of Cr2, using the auxiliary-field quantum Monte Carlo (AFQMC) method.

Stability, energetics, and magnetic states of cobalt adatoms on graphene.

We investigate the stability and electronic properties of single Co atoms on graphene with near-exact many-body calculations. A frozen-orbital embedding scheme was combined with auxiliary-field quantum Monte Carlo calculations to increase the reach in system sizes. Several energy minima are found as a function of the distance h between Co and graphene.

Frozen-Orbital and Downfolding Calculations with Auxiliary-Field Quantum Monte Carlo.

We describe the implementation of the frozen-orbital and downfolding approximations in the auxiliary-field quantum Monte Carlo (AFQMC) method. These approaches can provide significant computational savings, compared to fully correlating all of the electrons. While the many-body wave function is never explicit in AFQMC, its random walkers are Slater determinants, whose orbitals may be expressed in terms of any one-particle orbital basis.

Assessing weak hydrogen binding on Ca+ centers: an accurate many-body study with large basis sets.

Weak H(2) physisorption energies present a significant challenge to even the best correlated theoretical many-body methods. We use the phaseless auxiliary-field quantum Monte Carlo method to accurately predict the binding energy of Ca(+)-4H(2). Attention has recently focused on this model chemistry to test the reliability of electronic structure methods for H(2) binding on dispersed alkaline earth metal centers.

First-principles calculations of 17O nuclear magnetic resonance chemical shielding in Pb(Zr(1/2)Ti(1/2))O3 and Pb(Mg(1/3)Nb(2/3))O3: linear dependence on transition-metal/oxygen bond lengths.

First-principles density functional theory oxygen chemical shift tensors were calculated for A(B,B')O(3) perovskite alloys Pb(Zr(1/2)Ti(1/2))O(3) (PZT) and Pb(Mg(1/3)Nb(2/3))O(3) (PMN). Quantum chemistry methods for embedded clusters and the gauge including projector augmented waves (GIPAW) method [C. J.

High sensitivity of 17O NMR to p-d hybridization in transition metal perovskites: first principles calculations of large anisotropic chemical shielding.

A first principles embedded cluster approach is used to calculate O chemical shielding tensors, sigma, in prototypical transition metal oxide ABO(3) perovskite crystals. Our principal findings are (1) a large anisotropy of sigma between deshielded sigma(x) approximately sigma(y) and shielded sigma(z) components (z along the Ti-O bond); (2) a nearly linear variation, across all the systems studied, of the isotropic sigma(iso) and uniaxial sigma(ax) components, as a function of the B-O-B bond asymmetry. We show that the anisotropy and linear variation arise from large paramagnetic contributions to sigma(x) and sigma(y) due to virtual transitions between O(2p) and unoccupied B(nd) states.

Excited state calculations using phaseless auxiliary-field quantum Monte Carlo: Potential energy curves of low-lying C(2) singlet states.

We show that the recently developed phaseless auxiliary-field quantum Monte Carlo (AFQMC) method can be used to study excited states, providing an alternative to standard quantum chemistry methods. The phaseless AFQMC approach, whose computational cost scales as M(3)-M(4) with system size M, has been shown to be among the most accurate many-body methods in ground state calculations. For excited states, prevention of collapse into the ground state and control of the Fermion sign/phase problem are accomplished by the approximate phaseless constraint with a trial wave function.

Finite-size correction in many-body electronic structure calculations.

Finite-size (FS) effects are a major source of error in many-body (MB) electronic structure calculations of extended systems. A method is presented to correct for such errors. We show that MB FS effects can be effectively included in a modified local density approximation calculation.

Eliminating spin contamination in auxiliary-field quantum Monte Carlo: realistic potential energy curve of F(2).

The use of an approximate reference state wave function mid R:Phi(r) in electronic many-body methods can break the spin symmetry of Born-Oppenheimer spin-independent Hamiltonians. This can result in significant errors, especially when bonds are stretched or broken. A simple spin-projection method is introduced for auxiliary-field quantum Monte Carlo (AFQMC) calculations, which yields spin-contamination-free results, even with a spin-contaminated mid R:Phi(r).

Bond breaking with auxiliary-field quantum Monte Carlo.

Bond stretching mimics different levels of electron correlation and provides a challenging test bed for approximate many-body computational methods. Using the recently developed phaseless auxiliary-field quantum Monte Carlo (AF QMC) method, we examine bond stretching in the well-studied molecules BH and N(2) and in the H(50) chain. To control the sign/phase problem, the phaseless AF QMC method constrains the paths in the auxiliary-field path integrals with an approximate phase condition that depends on a trial wave function.

A study of H+H2 and several H-bonded molecules by phaseless auxiliary-field quantum Monte Carlo with plane wave and Gaussian basis sets.

The authors present phaseless auxiliary-field (AF) quantum Monte Carlo (QMC) calculations of the ground states of some hydrogen-bonded systems. These systems were selected to test and benchmark different aspects of the new phaseless AF QMC method. They include the transition state of H+H(2) near the equilibrium geometry and in the van der Walls limit, as well as the H(2)O, OH, and H(2)O(2) molecules.

Auxiliary-field quantum Monte Carlo study of first- and second-row post-d elements.

A series of calculations for the first- and second-row post-d elements (Ga-Br and In-I) are presented using the phaseless auxiliary-field quantum Monte Carlo (AF QMC) method. This method is formulated in a Hilbert space defined by any chosen one-particle basis and maps the many-body problem into a linear combination of independent-particle solutions with external auxiliary fields. The phase/sign problem is handled approximately by the phaseless formalism using a trial wave function, which in our calculations was chosen to be the Hartree-Fock solution.

Auxiliary-field quantum Monte Carlo calculations of molecular systems with a Gaussian basis.

We extend the recently introduced phaseless auxiliary-field quantum Monte Carlo (QMC) approach to any single-particle basis and apply it to molecular systems with Gaussian basis sets. QMC methods in general scale favorably with the system size as a low power. A QMC approach with auxiliary fields, in principle, allows an exact solution of the Schrodinger equation in the chosen basis.

Assessing and using the multiple correlated components of the burden of disease in decision-making in health care.

In medical decision-making, we must (a) establish the patient's prognosis, and (b) identify the therapeutic strategy that will best improve that prognosis. The prognosis is the projected progression over time of the cumulative probability of (a) mortality, (b) morbidity, (c) disability, (d) psychological distress, and (e) resource use. Our modeling employs parametric representations and multivariate analysis to produce the predicted individual and joint probabilities of any range of each measure and of combinations of the measures.

Projected survival benefit as criterion for listing and organ allocation in heart transplantation.

Background: Current policies for the selection of candidates and the allocation of hearts for transplantation give priority to patients at greatest risk if not transplanted. However, to achieve best use of the donated organs, it is necessary to estimate the net benefit associated with transplantation.

Methods: The survival benefit associated with being listed or not, with being transplanted or left on the waiting list, or with being transplanted or being denied the opportunity for a transplant can be estimated by means of time-to-event modeling of competing risks with intervening states.

Beyond survival: the burden of disease in decision making in organ transplantation.

Organ transplantation has long been perceived as a life-saving intervention. However, it is now recognized that the broader objectives include reducing the patient's burden of disease. The components of the burden of disease include (i) mortality, (ii) morbidity, (iii) disability, (iv) psychological distress and (v) resource use.

Quantum Monte Carlo method using phase-free random walks with slater determinants.

We develop a quantum Monte Carlo method for many fermions using random walks in the space of Slater determinants. An approximate approach is formulated with a trial wave function |Psi(T)> to control the phase problem. Using a plane-wave basis and nonlocal pseudopotentials, we apply the method to Be, Si, and P atoms and dimers, and to bulk Si supercells.

Time-to-event modeling of competing risks with intervening states in transplantation.

The criteria for the selection of who among the persons on the waiting is to receive an organ that has become available and who is to be placed on the list to begin with are the most contentious issues in organ transplantation. The decisions of whom to list and whom to transplant should take into account the net benefit to the individual patient and to the affected group as a whole. We present a method to compute the survival benefit by means of fully parametric modeling of the competing events (transplantation, death while awaiting the transplant, removal for other reasons), taking into account the transplant as an intervening state on the path to death post-transplant, and apply it to decisions whether to list or not list and whether to transplant or to leave on the waiting list or to remove from the list.