Restricted Density Functional Theory Research Articles

Analysis of the physical contributions to magnetic couplings in broken symmetry density functional theory approach

We present a new and simple scheme that aims to decompose into its main physical contributions the magnetic exchange interaction between two unpaired electrons. It is based on the popular broken-symmetry density functional theory (DFT) approach and relies on the frozen orbital capabilities of the local self-consistent field method. Accordingly, the magnetic exchange interaction energy can be separated into three main contributions: the direct exchange between magnetic orbitals, the spin polarization of the core orbitals, and the relaxation of the magnetic orbitals (kinetic exchange). This decomposition scheme is applied to a series of binuclear inorganic magnetic compounds both ferromagnetic and antiferromagnetic. The direct exchange is determined from the restricted DFT description. On the one hand, starting from the restricted orbital set and relaxing only the magnetic orbitals provides the kinetic exchange contribution and an estimate of the t and U parameters of the generalized Anderson mechanism. On the other hand, relaxing the core orbitals only introduces the spin polarization contribution. The decomposition leads to almost additive contributions. The effect of the amount of Hartree-Fock exchange on the different contributions is analyzed.

Quantifying the Importance of Orbital Over Spin Correlations in δ-Pu Within Density-Functional Theory

AbstractThe electronic structure of plutonium is studied within the density-functional theory (DFT) model. Key features of the electronic structure are correctly modeled and bonding, total energy, and electron density of states are all consistent with measure data, although the prediction of magnetism is not consistent with many observations. Here we analyze the contributions to the electronic structure arising from spin polarization, orbital polarization, and spin-orbit interaction. These effects give rise to spin and orbital moments that are of nearly equal magnitude, but anti-parallel, suggesting a magnetic-moment cancellation with a zero total moment. Quantifying the spin versus orbital effects on the bonding, total energy, and electron spectra it becomes clear that the spin polarization is much less important than the orbital correlations. Consequently, a restricted DFT approach with a non-spin polarized electronic structure can produce reasonable equation-of-state and electron spectra for δ-Pu when the orbital effects are accounted for. Hence, we present two non-magnetic models. One in which the spin moment is canceled by the orbital moment and another in which the spin moment (and therefore the orbital moment) is restricted to zero.

A DFT Study of the Thermal, Orbital Symmetry Forbidden, Cyclophanediene to Dihydropyrene Electrocyclic Reaction. Predictions to Improve the Dimethyldihydropyrene Photoswitches

The orbital symmetry forbidden thermal electrocyclic equilibria between a series of cyclophanedienes and dimethyldihydropyrenes (CPD<==>DDPs) were studied using density functional theory (DFT). These reactions are important not only because of their fundamental interest but also in how they restrict the potential utility of the DDP photoswitches by limiting the thermal lifetime of the CPDs. The transition states (TSs) for these reactions could not be modeled using restricted DFT (RB3LYP) but were located using unrestricted DFT (UB3LYP). Each TS possesses significant biradical character as indicated by their spin contaminated wave functions, S2 not = 0. Specific substitution by nitrile or trifluoromethyl group(s) is predicted to strongly affect the magnitude of the activation barriers for these reactions. In particular, replacing the internal methyl groups of the CPDs/DDPs with nitrile groups is predicted to have the maximum effect and to raise the activation barriers and lifetimes of the CPDs considerably.

Time-dependent density-functional theory calculations of triplet-triplet absorption

We present density-functional theory calculations of triplet-triplet absorption by three different approaches based on time-dependent density-functional theory (DFT): unrestricted DFT linear response, open-shell restricted DFT linear response applied to the triplet state, and quadratic response with triplet excitations applied to the ground state. Comparison is also made with corresponding results obtained by Hartree-Fock and multiconfiguration self-consistent-field response theory. Two main conclusions concerning triplet-triplet transitions are drawn in this study: First, the very good agreement between unrestricted and restricted DFT results indicates that spin contamination of the triplet state is not a serious problem when computing triplet-triplet spectra of common organic molecules. Second, DFT response calculations of triplet-triplet transitions can be affected by triplet instability problems, especially for the combination of DFT quadratic response with functionals containing fractional exact Hartree-Fock exchange.

Are both symmetric and buckled dimers on Si(100) minima? Density functional and multireference perturbation theory calculations

We report a spin-unrestricted density functional theory (DFT) solution at the symmetric dimer structure for cluster models of Si(100). With this solution, it is shown that the symmetric structure is a minimum on the DFT potential energy surface, although higher in energy than the buckled structure. In restricted DFT calculations the symmetric structure is a saddle point connecting the two buckled minima. To further assess the effects of electron correlation on the relative energies of symmetric versus buckled dimers on Si(100), multireference second order perturbation theory (MRMP2) calculations are performed on these DFT optimized minima. The symmetric structure is predicted to be lower in energy than the buckled structure via MRMP2, while the reverse order is found by DFT. The implications for recent experimental interpretations are discussed.

Some thoughts about the stability and reliability of commonly used exchange?correlation functionals ? coverage of dynamic and nondynamic correlation effects

The self-interaction error (SIE) of commonly used density functional theory (DFT) exchange functionals mimics long-range (nondynamic) pair correlation effects in an unspecified way. Slater exchange suffers from a larger SIE and, therefore, covers more nondynamic correlation effects than Becke exchange, which is the reason why exchange–correlation (XC) functionals based on Slater exchange lead to stabler restricted DFT solutions than those based on Becke exchange. However, the stability of an XC functional does not guarantee higher accuracy. On the contrary, if system-specific nondynamic correlation effects have to be introduced via the form of the wave function, these will be suppressed by nondynamic correlation effects already covered by the exchange functional. Hybrid functionals suffer less from the SIE and, therefore, cover a smaller number of nondynamic electron correlation effects. Accordingly, they are better suited when nondynamic electron correlation has to be introduced by the form of the wave function. It is shown that, for example, broken-symmetry unrestricted DFT calculations are more accurate when carried out with B3LYP than BLYP contrary to claims made in the literature.

Benzdiynes revisited: ab initio and density functional theory

Ab initio and density functional theory (DFT) studies were performed on three isomers of tetradehydrobenzene (benzdiynes). Four different density functionals (BPW91, BLYP, B3LYP, and B1LYP) and two higher levels of theory [QCISD and CCSD(T)] incorporating basis sets up to Dunning's correlation-consistent polarized valence triple-ζ (cc-pVTZ) were utilized for this purpose. Stability tests showed that more stable solutions were available for 1,4-benzdiyne with unrestricted than with restricted DFT, while solutions obtained with later descriptions of 1,3-benzdiyne and 1,2,3,5-tetradehydrobenzene were stable. UB3LYP provided better geometry for 1,4-benzdiyne. Unlike in an earlier study, the energy difference between 1,2,3,5-tetradehydrobenzene and 1,4-benzdiyne calculated with the B3LYP functional coincided well with that calculated at CCSD(T): 13.07 and 14.32 kcal/mol at the B3LYP/cc-pVTZ and CCSD(T)/cc-pVTZ levels, respectively. 1,2,3,5-Tetradehydrobenzene was 8 kcal/mol more stable than 1,3-benzdiyne at the CCSD(T)/cc-pVTZ level. The heats of formation of benzdiynes were obtained by using the G2, CBS-Q, and CBS-QB3 methodologies. The heats of formation calculated for 1,3-benzdiyne and 1,2,3,5-tetradehydrobenzene were 208.6 and 197.9 kcal/mol, respectively, at the CBS-QB3 level of theory. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 923–930, 2001

Benzdiynes revisited: ab initio and density functional theory

AbstractAb initio and density functional theory (DFT) studies were performed on three isomers of tetradehydrobenzene (benzdiynes). Four different density functionals (BPW91, BLYP, B3LYP, and B1LYP) and two higher levels of theory [QCISD and CCSD(T)] incorporating basis sets up to Dunning's correlation‐consistent polarized valence triple‐ζ (cc‐pVTZ) were utilized for this purpose. Stability tests showed that more stable solutions were available for 1,4‐benzdiyne with unrestricted than with restricted DFT, while solutions obtained with later descriptions of 1,3‐benzdiyne and 1,2,3,5‐tetradehydrobenzene were stable. UB3LYP provided better geometry for 1,4‐benzdiyne. Unlike in an earlier study, the energy difference between 1,2,3,5‐tetradehydrobenzene and 1,4‐benzdiyne calculated with the B3LYP functional coincided well with that calculated at CCSD(T): 13.07 and 14.32 kcal/mol at the B3LYP/cc‐pVTZ and CCSD(T)/cc‐pVTZ levels, respectively. 1,2,3,5‐Tetradehydrobenzene was 8 kcal/mol more stable than 1,3‐benzdiyne at the CCSD(T)/cc‐pVTZ level. The heats of formation of benzdiynes were obtained by using the G2, CBS‐Q, and CBS‐QB3 methodologies. The heats of formation calculated for 1,3‐benzdiyne and 1,2,3,5‐tetradehydrobenzene were 208.6 and 197.9 kcal/mol, respectively, at the CBS‐QB3 level of theory. © 2001 John Wiley &amp; Sons, Inc. J Comput Chem 22: 923–930, 2001