Nicholsonjefferson0410

abinit is probably the first electronic-structure package to have been released under an open-source license about 20 years ago. It implements density functional theory, density-functional perturbation theory (DFPT), many-body perturbation theory (GW approximation and Bethe-Salpeter equation), and more specific or advanced formalisms, such as dynamical mean-field theory (DMFT) and the "temperature-dependent effective potential" approach for anharmonic effects. Relying on planewaves for the representation of wavefunctions, density, and other space-dependent quantities, with pseudopotentials or projector-augmented waves (PAWs), it is well suited for the study of periodic materials, although nanostructures and molecules can be treated with the supercell technique. The present article starts with a brief description of the project, a summary of the theories upon which abinit relies, and a list of the associated capabilities. It then focuses on selected capabilities that might not be present in the majority of electronic structure packages either among planewave codes or, in general, treatment of strongly correlated materials using DMFT; materials under finite electric fields; properties at nuclei (electric field gradient, Mössbauer shifts, and orbital magnetization); positron annihilation; Raman intensities and electro-optic effect; and DFPT calculations of response to strain perturbation (elastic constants and piezoelectricity), spatial dispersion (flexoelectricity), electronic mobility, temperature dependence of the gap, and spin-magnetic-field perturbation. The abinit DFPT implementation is very general, including systems with van der Waals interaction or with noncollinear magnetism. Community projects are also described generation of pseudopotential and PAW datasets, high-throughput calculations (databases of phonon band structure, second-harmonic generation, and GW computations of bandgaps), and the library libpaw. abinit has strong links with many other software projects that are briefly mentioned.Developing a computational method that is both affordable and accurate for transition-metal chemistry is a major challenge. The bond dissociation energies and the potential energy curves are two important targets for theoretical prediction. Here, we investigate the performance of multiconfiguration pair-density functional theory (MC-PDFT) based on wave functions calculated by the complete-active-space (CAS) and generalized active space (GAS) self-consistent-field (SCF) methods for three transition-metal diatomics (TiC, TiSi, and WCl) for which accurate bond energies are available from recent experiments. We compare the results to those obtained by CAS second-order perturbation theory (CASPT2) and Kohn-Sham DFT (KS-DFT). We use six systematic methods to choose the active spaces (1) we put the bonding orbitals, antibonding orbitals, and singly occupied nonbonding orbitals into the active space in the first method; (2) we also put s and p valence orbitals into the active space; we tried two levels of correlated participating orbitals (CPO) active spaces (3) nominal CPO (nom-CPO) and (4) extended CPO (ext-CPO); and we used (5) the separated-pair (SP) approximation and (6) a new method presented here called extended separate pairs (ESP) approximation to divide the nom-CPO active space into subspaces. Schemes 1-4 are carried out within the CAS framework, and schemes 5 and 6 are carried out in the GAS framework to eliminate deadwood configurations. selleck chemical For TiC and TiSi, we used all six kinds of active spaces. For WCl, we used three active spaces (nom-CPO, SP, and ESP). We found that MC-PDFT performs better than both CASPT2 and KS-DFT. We also found that the SP (for TiSi) and ESP (for TiC and WCl) approximations are particularly appealing because they make the potential curves smoother and significantly decrease the computational cost of CASSCF calculations. Furthermore, ESP-PDFT can be as accurate as CAS-PDFT.X-ray absorption spectroscopy measurements were performed for the C K-edge of Pt nanoparticles on Ar+-irradiated carbon supports in order to elucidate the origin of improved catalyst performance after the introduction of vacancies into the carbon support. We observed a change in the electronic structure at the interface between the Pt nanoparticles and the carbon support after vacancy introduction, which is in good agreement with theoretical results. The results indicated that vacancy introduction resulted in a drastic change in the Pt-C interactions, which likely affected the d-band center of the Pt nanoparticles and led to the enhancement of the oxygen reduction reaction in catalysts.We investigate performance of the equation-of-motion coupled-cluster method at the single and doubles level (EOM-CCSD) and a series of approximate methods based on EOM-CCSD on electron affinities (EA) of closed-shell cations and neutral molecules with positive and negative EAs in this work. Our results confirm that P-EOM-MBPT2 can provide reasonable EAs when molecules with significant multireference character are not considered and its mean absolute error on EAs of these molecules is around or less than 0.2 eV. Its accuracy is comparable to that of the more expensive EOM-CCSD(2) method. Results of EOM-CCSD(2), P-EOM-MBPT2, and CIS(D∞) indicate that the [[H, ac +], T2] term in the 1h2p-1h block is more important on EAs than the term neglected in the 1h2p-1h2p block in P-EOM-MBPT2. We proposed an economical method where EAs from CIS(D∞) are corrected by treating this [[H, ac +], T2] term in the 1h2p-1h block perturbatively [corr-CIS(D∞)]. EAs with corr-CIS(D∞) agree very well with those of P-EOM-MBPT2 with a difference of less than 0.02 eV. Computational scaling of this method is N4 for the iterative part and N5 for some non-iterative steps. Its storage requirement is only of OV3. Corr-CIS(D∞) is an economical and reliable method on EAs, and it can be applied to EAs of large molecules.Matrix effects, which cause a change in ion intensity, occur in mass spectrometry methods including time-of-flight secondary ion mass spectrometry (TOF-SIMS). Matrix effects often cause large issues in quantitative analysis because secondary ions related to a particular molecule could be dramatically enhanced or suppressed regardless of the concentration. To investigate matrix effects in biological samples, the authors evaluated mixed lipid POPC [1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine, molecular weight (MW) 759.6], peptide [leu-enkephalin, neo-leu-enkephalin (amino acid sequence YAGFL, MW 569.3), and neo-angiotensin II (amino acid sequence DRVYIHAF, MW 1019.5)] samples. Matrix effect features were investigated by analyzing the concentration dependence of secondary ions in lipid-peptide mixed samples to develop a method that enables quantitative analysis using TOF-SIMS. Matrix effects depended on the lipid-peptide combination. Interestingly, some secondary ions possessed an intensity that was highly dependent on concentration.