The ab-initio density matrix renormalization group (DMRG) is a tool that can be applied to a wide variety of interesting problems in quantum chemistry. Here, we examine the density matrix renormalization group from the vantage point of the quantum chemistry user. What kinds of problems is the DMRG well-suited to? What are the largest systems that can be treated at practical cost? What sort of accuracies can be obtained, and how do we reason about the computational difficulty in different molecules? By examining a diverse benchmark set of molecules: π-electron systems, benchmark main-group and transition metal dimers, and the Mn-oxo-salen and Fe-porphine organometallic compounds, we provide some answers to these questions, and show how the density matrix renormalization group is used in practice. C 2015 AIP Publishing LLC. [http://dx

Computation of lattice energies to an accuracy sufficient to distinguish polymorphs is a fundamental bottleneck in crystal structure prediction. For the lattice energy of the prototypical benzene crystal, we combined the quantum chemical advances of the last decade to attain sub-kilojoule per mole accuracy, an order-of-magnitude improvement in certainty over prior calculations that necessitates revision of the experimental extrapolation to 0 kelvin. Our computations reveal the nature of binding by improving on previously inaccessible or inaccurate multibody and many-electron contributions and provide revised estimates of the effects of temperature, vibrations, and relaxation. Our demonstration raises prospects for definitive first-principles resolution of competing polymorphs in molecular crystal structure prediction.

No abstract

The strongly-contracted variant of second order N-electron valence state perturbation theory (NEVPT2) is an efficient perturbative method to treat dynamic correlation without the problems of intruder states or level shifts, while the density matrix renormalization group (DMRG) provides the capability to tackle static correlation in large active spaces. We present a combination of the DMRG and strongly-contracted NEVPT2 (DMRG-SC-NEVPT2) that uses an efficient algorithm to compute high order reduced density matrices from DMRG wave functions. The capabilities of DMRG-SC-NEVPT2 are demonstrated on calculations of the chromium dimer potential energy curve at the basis set limit, and the excitation energies of poly-p-phenylene vinylene trimer (PPV(n=3)).

We describe and extend the formalism of state-specific analytic density matrix renormalization group (DMRG) energy gradients, first used by Liu et al. [J. Chem. Theor. Comput. 2013, 9, 4462]. We introduce a DMRG wave function maximum overlap following technique to facilitate state-specific DMRG excited-state optimization. Using DMRG configuration interaction (DMRG-CI) gradients, we relax the low-lying singlet states of a series of trans-polyenes up to C20H22. Using the relaxed excited-state geometries, as well as correlation functions, we elucidate the exciton, soliton, and bimagnon ("single-fission") character of the excited states, and find evidence for a planar conical intersection.

No abstract

No abstract

Background:The incidence of tumors located in the upper third of the stomach is increasing, and the use of radical proximal gastrectomy is becoming prevalent. After a proximal gastrectomy, various reconstructions are performed, but surgical outcomes are controversial. This study was performed to review clinical outcomes of reconstructions after proximal gastrectomy.Methods:Inclusion criteria focused on postoperative complications of patients who underwent a proximal gastrectomy for gastric cancer. Exclusion criteria were case reports; targeted data not investigated; a duplicate study reported in a larger cohort; esophageal sphincter preservation surgery; near-total gastrectomy; recurrence of tumor; and combined organ resection.Results:In total, 22 retrospective and 2 prospective studies were included. The studies investigated surgical outcomes of esophagogastrostomy (n = 10), jejunal interposition (n = 12), jejunal pouch interposition (n = 7), double tract jejunal interposition (n = 1), and tube-like stomach esophagogastrostomy (n = 5). Pooled incidences of reflux esophagitis or reflux symptoms for these procedures were 28.6%, 4.5%, 12.9%, 4.7%, and 10.7%, respectively. Incidences of postoperative complications were 9.5%, 18.1%, 7.0%, 11.6%, and 9.3%, respectively.Conclusions:Despite increasing operation complexity, which perhaps increased the risk of other postoperative complications, currently used reconstructions present excellent anti-reflux efficacy. However, the optimal reconstruction method remains to be determined.