Showing papers by "Christof Hättig published in 2013"

Benchmarks for 0–0 transitions of aromatic organic molecules: DFT/B3LYP, ADC(2), CC2, SOS-CC2 and SCS-CC2 compared to high-resolution gas-phase data

Abstract: In the present study a benchmark set of medium-sized and large aromatic organic molecules with 10–78 atoms is presented. For this test set 0–0 transition energies measured in supersonic jets are compared to those calculated with DFT and the B3LYP functional, ADC(2), CC2 and the spin-scaled CC2 variants SOS-CC2 and SCS-CC2. Geometries of the ground and excited states have been optimized with these methods in polarized triple zeta basis sets. Zero-point vibrational corrections have been calculated with the same methods and basis sets. In addition the energies have been corrected by single point calculations with a triple zeta basis augmented with diffuse functions, aug-cc-pVTZ. The deviations of the theoretical results from experimental electronic origins, which have all been measured in the gas phase with high-resolution techniques, were evaluated. The accuracy of SOS-CC2 is comparable to that of unscaled CC2, whereas ADC(2) has slightly larger errors. The lowest errors were found for SCS-CC2. All correlated wave function methods provide significantly better results than DFT with the B3LYP functional. The effects of the energy corrections from the augmented basis set and the method-consistent calculation of the zero-point vibrational corrections are small. With this benchmark set reliable reference data for 0–0 transition energies for larger organic chromophores are available that can be used to benchmark the accuracy of other quantum chemical methods such as new DFT functionals or semi-empirical methods for excitation energies and structures and thereby augments available benchmark sets augments present benchmark sets which include mainly smaller molecules.

A pair natural orbital implementation of the coupled cluster model CC2 for excitation energies.

Abstract: We demonstrate how to extend the pair natural orbital (PNO) methodology for excited states, presented in a previous work for the perturbative doubles correction to configuration interaction singles (CIS(D)), to iterative coupled cluster methods such as the approximate singles and doubles model CC2. The original O(N5) scaling of the PNO construction is reduced by using orbital-specific virtuals (OSVs) as an intermediate step without spoiling the initial accuracy of the PNO method. Furthermore, a slower error convergence for charge-transfer states is analyzed and resolved by a numerical Laplace transformation during the PNO construction, so that an equally accurate treatment of local and charge-transfer excitations is achieved. With state-specific truncated PNO expansions, the eigenvalue problem is solved by combining the Davidson algorithm with deflation to project out roots that have already been determined and an automated refresh with a generation of new PNOs to achieve self-consistency of the PNO space...

A scaling PNO–MP2 method using a hybrid OSV–PNO approach with an iterative direct generation of OSVs†

Abstract: We present an implementation of pair natural orbital second-order Moller–Plesset perturbation theory with computational costs that scale only cubically with the system size. The low cost-scaling is achieved by combining a hybrid approach, where the pair natural orbitals are build from orbital-specific virtuals OSVs, with an iterative block Davidson algorithm for solving the equations for OSVs. We thereby avoid a complete diagonalisation of amplitude matrices and the explicit construction of the corresponding exchange integral matrices. This reduces the cost-scaling for the generation of the OSVs and of the pair natural orbitals to without a priori assumptions about locality. The costs can be further reduced by combining the approach with a local resolution-of-the-identity approximation for the exchange integrals. The errors introduced by these approximations are negligible and do not affect the final accuracy of the correlation energy. Test calculations on a set of organic and inorganic molecules demonstr...

Pair natural orbitals in explicitly correlated second-order moller-plesset theory

Abstract: An improved formulation of pair natural orbital (PNO)–based explicitly correlated Moller–Plesset second-order perturbation theory is presented. Increased efficiency has been achieved by modifying the representation of the strong orthogonality projector used in the evaluation of the F12 intermediates. We demonstrate two simple relationships between the truncation errors arising from PNOs in the virtual space and those arising from the PNOs in the strong orthogonality operators. This permits robust error control through a single parameter.

Computational screening of one- and two-photon spectrally tuned channelrhodopsin mutants

Abstract: Optogenetics is by now a well-established field within neuroscience where neuro-response is controlled at the molecular level using the photochemical properties of channelrhodopsin (ChR). In this study the recently published X-ray structure of retinal inside the ChR binding pocket serves as the basis for conducting high-level polarizable embedding quantum mechanical/molecular mechanical (QM/MM) mutation studies with the aim of providing insight into the tuning mechanisms of this remarkable protein. The levels of theory applied are the recently developed PERI-CC2 and PE-DFT approaches. Their computational efficiency makes it possible to rapidly carry out a large number of spectral calculations. This is exploited to construct in silico mutated ChR variants which are characterized in terms of the location of the relevant excitation energy and the magnitude of the two-photon absorption cross section. In turn, this allows us to pinpoint the amino acids that have the largest electrostatic effect on the studied excited state properties. We show that a single/double site mutation strategy in ChR does not perturb the electronic properties of retinal to a degree that satisfies the experimental demand for a significant red-shift. With respect to non-linear absorption we conjecture that the recently synthesized ChETA variant possesses an even larger two-photon cross section than the C1C2 variant and it is thus an ideal candidate for further studies on the two-photon activation of ChR.

A combined experimental and computational study on the adsorption and reactions of NO on rutile TiO2

Abstract: In this work we combined computational density functional theory with experimental infrared spectroscopy to determine the adsorbate structure of NO and its reaction products N2O2, N2O, and NO2 on rutile TiO2. These reactions are important for the photo-catalytic reduction of NO in exhaust gas, but yet little is known about the mechanisms or the intermediates involved. The combination of high-quality ultrahigh vacuum FTIRS data with large scale embedded cluster calculations using an accurate hybrid density functional rendered it possible to identify and assign unambiguously vibrational frequencies for nine species which are formed upon adsorption and reaction of NO on rutile TiO2. Some of them have been observed for the first time. As a result of the quantum chemical calculations we can report for all adsorbates accurate structures and binding energies.

Analytic Molecular Hessian Calculations for CC2 and MP2 Combined with the Resolution of Identity Approximation.

Abstract: An implementation of analytic second derivatives for the approximate coupled cluster singles and doubles model CC2 and for second-order Moller-Plesset perturbation theory (MP2) will be presented. The RI approximation for the two-electron repulsion integrals is used to reduce memory demands, operation count, and I/O requirements. During the calculation, the storage of [Formula: see text] quantities (where [Formula: see text] is a measure for the system size) can completely be avoided. It is shown that with the MP2 method and an appropriate scaling of the harmonic frequencies, especially C-F stretch frequencies are reproduced much better in comparison to experiments than with the B3LYP density functional. Similar advantages are observed for molecules with strong, internal van der Waals interactions. Spin scaling offers additional improvements in these cases. The implementation has been tested for molecules with up to 81 atoms and 684 basis functions.