CASSCF and MR–CISD study of the n−4s and n−4pe Rydberg states of the CF3Cl

AbstractThe low-lying n−4s (S2 and T2) and n−4pe (S3 and T3) Rydberg states of the CF3Cl have been studied at the CASSCF, MR–CISD, MR–CISD+Q and MR–AQCC levels using the mixed aug–cc–pVDZ/d–aug–cc–pVDZ and aug′–cc–pVTZ/d′–aug′–cc–pVTZ basis sets. Spin–orbit corrections for the singlet energies, vertical excitation energies and oscillator strengths have been computed. The effect of the inclusion of K+L or only the K inner shell chlorine orbitals in the frozen core space at the post-CASSCF levels has also been discussed. Good agreement with available experimental and with previous high-level theoretical results has been achieved

The generality of the GUGA MRCI approach in COLUMBUS for treating complex quantum chemistry

The core part of the program system COLUMBUS allows highly efficient calculations using variational multireference (MR) methods in the framework of configuration interaction with single and double excitations (MR-CISD) and averaged quadratic coupled-cluster calculations (MR-AQCC), based on uncontracted sets of configurations and the graphical unitary group approach (GUGA). The availability of analytic MR-CISD and MR-AQCC energy gradients and analytic nonadiabatic couplings for MR-CISD enables exciting applications including, e.g., investigations of π-conjugated biradicaloid compounds, calculations of multitudes of excited states, development of diabatization procedures, and furnishing the electronic structure information for on-the-fly surface nonadiabatic dynamics. With fully variational uncontracted spin-orbit MRCI, COLUMBUS provides a unique possibility of performing high-level calculations on compounds containing heavy atoms up to lanthanides and actinides. Crucial for carrying out all of these calculations effectively is the availability of an efficient parallel code for the CI step. Configuration spaces of several billion in size now can be treated quite routinely on standard parallel computer clusters. Emerging developments in COLUMBUS, including the all configuration mean energy multiconfiguration self-consistent field method and the graphically contracted function method, promise to allow practically unlimited configuration space dimensions. Spin density based on the GUGA approach, analytic spin-orbit energy gradients, possibilities for local electron correlation MR calculations, development of general interfaces for nonadiabatic dynamics, and MRCI linear vibronic coupling models conclude this overview

A importância do método de Hartree no ensino de química quântica

Hartree's original ideas are described. Its connection with electrostatics can be explored in order to decrease the gap between teaching of Physics and Chemistry. As a consequence of its simplicity and connection with electrostatics, it is suggested that Hartree's method should be presented before the Hartree-Fock method. Besides, since the fundamental concepts of indistinguishibility of electrons along with the antissimetry of the wave function are missing in the Hartree's product, the method itself can be used to introduce these concepts. Despite the fact that these features are not included in the trial wavefunction, important qualitatively correct results can be obtained

Dissociation of ground and nσ* states of CF3Cl using multireference configuration interaction with singles and doubles and with multireference average quadratic coupled cluster extensivity corrections

Extended complete active space self-consistent field (CASSCF), multireference configuration interaction with singles and doubles (MR-CISD), and multireference average quadratic coupled cluster (MR-AQCC) calculations have been performed on the ground (S0) and first excited (nσ*,S1) states of the CF3Cl molecule. Full geometry optimizations have been carried out for S0 as well as “relaxed” potential energy calculations for both states, along the C–Cl bond distance. Vertical excitation energies (ΔEvertical), dissociation energies (ΔEdiss), dissociation enthalpies (ΔHdiss), and the oscillator strength (f) have also been computed. Basis set effects, basis set superposition error (BSSE), and spin-orbit and size-extensivity corrections have also been considered. The general agreement between theoretical and available experimental results is very good. The best results for the equilibrium geometrical parameters of S0 (at MR-AQCC/aug-cc-pVTZ+d level) are 1.762 and 1.323 Å, for the C–Cl and C–F bond distances, respectively, while the corresponding experimental values are 1.751 and 1.328 Å. The ∠ClCF and ∠FCF bond angles are in excellent agreement with the corresponding experimental values (110.3° and 108.6°). The best calculated values for ΔEvertical, ΔHdiss, and f are 7.63 eV [at the MR-AQCC/aug-cc-pV(T+d)Z level], 3.59 eV[MR-AQCC/aug-cc-pV(T+d)Z level+spin-orbit and BSSE corrections], and 2.74×10−3 (MR-CISD/cc-pVTZ), in comparison with the corresponding experimental values of 7.7±0.1 eV, 3.68 eV, and 3.12×10−3±2.50×10−4. The results concerning the potential energy curves for S0 and S1 show a tendency toward the nonoccurrence of crossing between these two states (in the intermediate region along the C–Cl coordinate), as the basis set size increases. Such tendency is accompanied by a decreasing well depth for the S1 state. Dynamic electronic correlation (especially at the MR-AQCC level) is also an important factor toward an absence of crossing along the C–Cl coordinate. Further investigations of a possible crossing using gradient driven techniques (at CASSCF and MR-CISD levels) seem to confirm its absence

Non-Kasha fluorescence of pyrene emerges from a dynamic equilibrium between excited states

Pyrene fluorescence after a high-energy electronic excitation exhibits a prominent band shoulder not present after excitation at low energies. The standard assignment of this shoulder as a non-Kasha emission from the second-excited state (S2) has been recently questioned. To elucidate this issue, we simulated the fluorescence of pyrene using two different theoretical approaches based on the vertical convolution and nonadiabatic dynamics with nuclear ensemble approaches. To conduct the necessary nonadiabatic dynamics simulations with high-lying electronic states and deal with fluorescence timescales of about 100 ns of this large molecule, we developed new computational protocols. The results from both approaches confirm that the band shoulder is, in fact, due to S2 emission. We show that the non-Kasha behavior is a dynamic-equilibrium effect, not caused by a metastable S2 minimum. However, it requires considerable vibrational energy, which can only be achieved in collisionless regimes after transitions into highly excited states. This strict condition explains why the S2 emission was not observed in some experiments

Quantification of the ionic character of multiconfigurational wave functions: the <i>Q</i>_a^t diagnostic

The complete active space self-consistent field (CASSCF) method is a cornerstone in modern excited-state quantum chemistry providing the starting point for most common multireference computations. However, CASSCF, when used with a minimal active space, can produce significant errors (>2 eV) even for the excitation energies of simple hydrocarbons if the states of interest possess ionic character. After illustrating this problem in some detail, we present a diagnostic for ionic character, denoted as Qat, that is readily computed from the transition density. A set of 11 molecules is considered to study errors in vertical excitation energies. State-averaged CASSCF obtains a mean absolute error (MAE) of 0.87 eV for the 34 singlet states considered. We highlight a strong correlation between the obtained errors and the Q at diagnostic, illustrating its power to predict problematic cases. Conversely, using multireference configuration interaction with single and double excitations and Pople’s size extensivity correction (MR-CISD+P), excellent results are obtained with an MAE of 0.11 eV. Furthermore, correlations with the Q at diagnostic disappear. In summary, we hope that the presented diagnostic will facilitate reliable and user-friendly multireference computations on conjugated organic molecules.</p

Challenges encountered during development of Mn porphyrin-based, potent redox-active drug and superoxide dismutase mimic, MnTnBuOE-2-PyP\u3csup\u3e5 +\u3c/sup\u3e, and its alkoxyalkyl analogues

We disclose here the studies that preceded and guided the preparation of the metal-based, redox-active therapeutic Mn(III) meso-tetrakis(N-n-butoxyethylpyridyl)porphyrin, MnTnBuOE-2-PyP5 + (BMX-001), which is currently in Phase I/II Clinical Trials at Duke University (USA) as a radioprotector of normal tissues in cancer patients. N-substituted pyridylporphyrins are ligands for Mn(III) complexes that are among the most potent superoxide dismutase mimics thus far synthesized. To advance their design, thereby improving their physical and chemical properties and bioavailability/toxicity profiles, we undertook a systematic study on placing oxygen atoms into N-alkylpyridyl chains via alkoxyalkylation reaction. For the first time we show here the unforeseen structural rearrangement that happens during the alkoxyalkylation reaction by the corresponding tosylates. Comprehensive experimental and computational approaches were employed to solve the rearrangement mechanism involved in quaternization of pyridyl nitrogens, which, instead of a single product, led to a variety of mixed N-alkoxyalkylated and N-alkylated pyridylporphyrins. The rearrangement mechanism involves the formation of an intermediate alkyl oxonium cation in a chain-length-dependent manner, which subsequently drives differential kinetics and thermodynamics of competing N-alkoxyalkylation versus in situ N-alkylation. The use of alkoxyalkyl tosylates, of different length of alkyl fragments adjacent to oxygen atom, allowed us to identify the set of alkyl fragments that would result in the synthesis of a single compound of high purity and excellent therapeutic potential