Case For Density Functional Theory Research Articles

A Systematic Exploration of B-F Bond Dissociation Enthalpies of Fluoroborane-Type Molecules at the CCSD(T)/CBS Level.

Fluoroborane-type molecules (R1R2B-F) are of interest in synthetic chemistry, but to date, apart from a handful of small species (such as H2BF, HBF2, and BF3), little is known concerning the effect of substituents in governing the strength of the B-F bonds of such species toward homolytic dissociation in the gas phase. In this study, we have calculated the bond dissociation enthalpies (BDEs) of thirty unique B-F bonds at the CCSD(T)/CBS level using the high-level W1w thermochemical protocol. The B-F bonds in all species considered are very strong, ranging from 545.9 kJ mol-1 in (H2B)2B-F to 729.2 kJ mol-1 HBF2. Nevertheless, these BDEs still vary over a wide range of 183.3 kJ mol-1. The structural properties that affect the BDEs are examined in detail, and the homolytic BDEs are rationalized based on molecule stabilization enthalpies and radical stabilization enthalpies. Since polar B-F bonds may represent a challenging test case for density functional theory (DFT) methods, we proceed to examine the performance of a wide range of DFT methods across the rungs of Jacob's Ladder for their ability to compute B-F BDEs. We find that only a handful of DFT methods can reproduce the CCSD(T)/CBS BDEs with mean absolute deviations (MADs) below the threshold of chemical accuracy (i.e., with average deviations below 4.2 kJ mol-1). The only functionals capable of achieving this feat were (MADs given in parentheses): ωB97M-V (4.0), BMK (3.5), DSD-BLYP (3.8), and DSD-PBEB95 (1.8 kJ mol-1).

Can DFT Accurately Predict Spin Densities? Analysis of Discrepancies in Iron Nitrosyl Complexes.

Iron nitrosyl complexes are a particularly challenging case for density functional theory. In particular, for the low-spin state, different exchange-correlation functionals yield very different spin densities [ Conradie , J. ; Ghosh , A. J. Phys. Chem. B 2007 , 111 , 12621 - 12624 ]. Here, we investigate the origin of these differences in detail by analyzing the Kohn-Sham molecular orbitals. Furthermore, to decide which exchange-correlation functionals yield the most accurate spin densities, we make comparisons to CASSCF calculations. To ensure that the spin densities are converged with respect to the size of the active space, this comparison is performed for [Fe(NO)](2+) as a model system. We find that none of the investigated exchange-correlation functionals are able to reproduce the CASSCF spin densities accurately.

Excitation Energies in Solution: The Fully Polarizable QM/MM/PCM Method

We present the theory and an implementation of the combined quantum mechanics/molecular mechanics/polarizable dielectric continuum (QM/MM/PCM) method. This is a fully polarizable layered model designed for effective inclusion of a medium in a quantum-mechanical calculation. The short-range part of the solvent electrostatic potential is described by an atomistic model while the long-range part of this potential is described by a dielectric continuum. The QM/MM/PCM method has been implemented in combination with QM linear response techniques allowing for the assessment of, e.g., vertical electronic excitation energies and linear dipole-dipole polarizabilities, in all cases using a nonequilibrium formulation of the environmental response. The model is general, but is here implemented for the case of density functional theory. Numerical examples are given for solvatochromic shifts relating to a set of organic molecules in aqueous solution. We find in general the QM/MM/PCM interface to exhibit a faster convergence with respect to the system size as compared to the use of QM/MM only.

Performances of recently-proposed functionals for describing disulfide radical anions and similar systems

Two-center three-electron anions have been reported as a pitfall case for density functional theory, owing to their important dynamical correlation and the inherent difficulty of a balanced description of odd- vs. even-number electron bonds. The critical roles of the correlation functional, and of the amount of exact Hartree–Fock exchange included are investigated in order to pinpoint the most efficient approach for simulating disulfide radical anions (ease of formation and inter-sulfur distances). Several recently-developed functionals, such as B1B95, BMK, B2PLYP(D) and numerous range-separated hybrid yield a quantitatively correct description, and offer a pragmatic and superior solution to the Becke’s half-and-half usually recommended for this delicate situation.

Prediction of spin-spin coupling constants in solution based on combined density functional theory/molecular mechanics

We present theory and implementation of calculation of spin-spin coupling constants within combined quantum mechanics/molecular mechanics methods. Special attention is given to the role of explicit solvent polarization as well as the molecular consequences due to hydrogen bonding. The model is generally applicable but is here implemented for the case of density functional theory. First applications to liquid water and acetylene in aqueous solution are presented. Good agreement between theory and experiment is obtained in both cases, thereby showing the strength of our approach. Finally, spin-spin coupling constants across hydrogen bonds are discussed considering for the first time the role of an explicit solvent on this class of spin-spin couplings.

Hybrid Meta-Generalized Gradient Functional Modeling of Boron-Nitrogen Coordinate Covalent Bonds.

Truhlar's new generation of hybrid meta-generalized gradient functionals has been evaluated in modeling the binding enthalpies of substituted B-N coordinate covalent bonds. The short-range exchange correlation (XC) energy of coordinate covalent bonding coupled with the medium-range XC energy of noncovalent interactions results in a particularly difficult case for density functional theory (DFT). In this study, M06, M06-2X, M05, M05-2X, MPWB1K, and MPW1B95 with the 6-311++G(3df,2p) basis set have been used to evaluate four methylated ammonia trimethylboranes, (CH3)3B-N(CH3)nH3-n (n = 0 to 3), along with H3B-NH3. The predicted binding enthalpies from the new functionals have been compared to experiment as well as previous DFT (B3LYP, MPW1K) and ab initio (HF, MP2, QCISD, and QCISD(T)) results. Previously, only MP2, QCISD, and QCISD(T) were found to model the experimental energetic trend accurately. The mean absolute deviation (MAD) from experimental binding enthalpies for M06-2X and M05-2X is 0.3 and 1.6 kcal/mol, respectively. M06-2X yields a lower MAD than more expensive ab initio methods (MP2 = 1.9 kcal/mol and QCISD = 2.3 kcal/mol) and a comparable MAD to QCISD(T) (MAD = 0.4 kcal/mol). M06-2X is shown to provide a balanced account of the short- and medium-range XC energies necessary to describe the binding enthalpy of coordinate covalent bonds accurately in sterically congested molecular systems.

Design of Density Functionals by Combining the Method of Constraint Satisfaction with Parametrization for Thermochemistry, Thermochemical Kinetics, and Noncovalent Interactions.

We present a new hybrid meta exchange-correlation functional, called M05-2X, for thermochemistry, thermochemical kinetics, and noncovalent interactions. We also provide a full discussion of the new M05 functional, previously presented in a short communication. The M05 functional was parametrized including both metals and nonmetals, whereas M05-2X is a high-nonlocality functional with double the amount of nonlocal exchange (2X) that is parametrized only for nonmetals. In particular, M05 was parametrized against 35 data values, and M05-2X is parametrized against 34 data values. Both functionals, along with 28 other functionals, have been comparatively assessed against 234 data values: the MGAE109/3 main-group atomization energy database, the IP13/3 ionization potential database, the EA13/3 electron affinity database, the HTBH38/4 database of barrier height for hydrogen-transfer reactions, five noncovalent databases, two databases involving metal-metal and metal-ligand bond energies, a dipole moment database, a database of four alkyl bond dissociation energies of alkanes and ethers, and three total energies of one-electron systems. We also tested the new functionals and 12 others for eight hydrogen-bonding and stacking interaction energies in nucleobase pairs, and we tested M05 and M05-2X and 19 other functionals for the geometry, dipole moment, and binding energy of HCN-BF3, which has recently been shown to be a very difficult case for density functional theory. We tested eight functionals for four more alkyl bond dissociation energies, and we tested 12 functionals for several additional bond energies with varying amounts of multireference character. On the basis of all the results for 256 data values in 18 databases in the present study, we recommend M05-2X, M05, PW6B95, PWB6K, and MPWB1K for general-purpose applications in thermochemistry, kinetics, and noncovalent interactions involving nonmetals and we recommend M05 for studies involving both metallic and nonmetallic elements. The M05 functional, essentially uniquely among the functionals with broad applicability to chemistry, also performs well not only for main-group thermochemistry and radical reaction barrier heights but also for transition-metal-transition-metal interactions. The M05-2X functional has the best performance for thermochemical kinetics, noncovalent interactions (especially weak interaction, hydrogen bonding, π···π stacking, and interactions energies of nucleobases), and alkyl bond dissociation energies and the best composite results for energetics, excluding metals.

Exchange and correlation interplay in density functional theory under test: therelative stability of C3H isomers

The isomerization barrier between the two most stable C3H isomers has beenconsidered in this work as a test case for density functional theory (DFT)calculations. A wide range of exchange and correlation functionals arechecked and their results are compared to very accurate state-of-the-artcoupled-cluster benchmark calculations. Special care is devoted to basis seteffects as well as to the role played separately by exchange and correlationenergies. A few exchange–correlation functionals analysed here are able toapproximate the exact result within the so-called ‘chemical accuracy’ (± 1 kcalmol−1) and some of them even reach the most stringent ‘calibration accuracy’ (± 1 kJmol−1). However, a warning message is also given, since the wide spread of theresults is large enough to prevent a general agreement between DFT and exactvalues.

The Failure of Generalized Gradient Approximations (GGAs) and Meta-GGAs for the Two-Center Three-Electron Bonds in He2+, (H2O)2+, and (NH3)2+

The radical cations He2+, (H2O)2+, and (NH3)2+ with two-center three-electron A−A bonds are investigated at the configuration interaction (CI), accurate Kohn−Sham (KS), generalized gradient approximation (GGA), and meta-GGA levels. Assessment of seven different GGA and six meta-GGA methods shows that the A2+ systems remain a difficult case for density functional theory (DFT). All methods tested consistently overestimate the stability of A2+: the corresponding De errors decrease for more diffuse valence densities in the series He2+ > (H2O)2+ > (NH3)2+. Upon comparison to the energy terms of the accurate Kohn−Sham solutions, the approximate exchange functionals are found to be responsible for the errors of GGA-type methods, which characteristically overestimate the exchange in A2+. These so-called exchange functionals implicitly use localized holes. Such localized holes do occur if there is left−right correlation, i.e., the exchange functionals then also describe nondynamical correlation. However, in the h...

The calculation of NMR shielding tensors based on density functional theory and the frozen-core approximation

The application of the frozen-core approximation to the calculation of the shielding tensor of nuclear magnetic resonance (NMR) spectroscopy is discussed and an implementation is presented. A complete formulation of the shielding calculation within the frozen-core approximation is given, both in general terms and for the special case of density functional theory (DFT) and “gauge including atomic orbitals” (GIAOs). The practical implementation is validated by a detailed discussion of the consequences of the approximation. The general conclusion is drawn that the frozen-core approximation is a useful tool for shielding calculations—if the valence space is increased to contain at least the ns, np, (n − 1)p, (n − 1)d (fourth period and higher) shells, where n is the number of the given period in the periodic table of elements. The new method is applied to 77Se shieldings and chemical shifts for a small number of compounds. The agreement between theory and experiment is good for relative shifts, whereas calculated absolute shieldings are generally too small by about 300–400 ppm. This difference is attributed to the relativistic contraction of the core density at the selenium atom that had been explicitly incorporated into the experimental absolute shielding scale. © 1996 John Wiley & Sons, Inc.