373 research outputs found
A perturbative approximation to DFT/MRCI: DFT/MRCI(2)
We introduce a perturbative approximation to the combined density functional
theory and multireference configuration interaction (DFT/MRCI) method. The
method, termed DFT/MRCI(2), results from the application of quasi-degenerate
perturbation theory and the Epstein-Nesbet partitioning of the DFT/MRCI
Hamiltonian matrix. This results in the replacement of the diagonalization of
the large DFT/MRCI Hamiltonian with that of a small effective Hamiltonian, and
affords orders of magnitude savings in terms of computational cost. Moreover,
the DFT/MRCI(2) approximation is found to be of excellent accuracy, furnishing
excitation energies with a root mean squared deviation from the DFT/MRCI values
of less than 0.03 eV for an extensive test set of organic molecules
Ultrafast X-ray spectroscopy of conical intersections
Ongoing developments in ultrafast X-ray sources offer powerful new means of
probing the com- plex non-adiabatically coupled structural and electronic
dynamics of photoexcited molecules. These non-Born-Oppenheimer effects are
governed by general electronic degeneracies termed conical in- tersections
which play a key role, analogous to that of a transition state, in the
electronic-nuclear dynamics of excited molecules. Using high level ab initio
quantum dynamics simulations, we studied time-resolved X-ray absorption and
photoelectron spectroscopy (TRXAS and TRXPS, respectively) of the prototypical
unsaturated organic chromophore, ethylene, following excitation to its S2
state. The TRXAS in particular is highly sensitive to all aspects of the
ensuing dynamics. These X-ray spectroscopies provide a clear signature of the
wavepacket dynamics near conical intersections, related to charge localization
effects driven by the nuclear dynamics. Given the ubiquity of charge
localization in excited state dynamics, we believe that ultrafast X-ray
spectroscopies offer a unique and powerful route to the direct observation of
dynamics around conical intersections.Comment: 5 pages, 4 figure
Design of 370-ps Delay Floating-Voltage Level Shifters With 30-V/ns Power Supply Slew Tolerance
A new design method for producing high-performance and power-rail slew-tolerant floating-voltage level shifters is presented, offering increased speed, reduced power consumption, and smaller layout area compared with previous designs. The method uses an energy-saving pulse-triggered input, a high-bandwidth current mirror, and a simple full latch composed of two inverters. A number of optimizations are explored in detail, resulting in a presented design with a dVdd slew immunity of 30 V/ns, and near-zero static power dissipation in a 180-nm technology. Experimental results show a delay of below 370 ps for a level-shift range of 8-20 V. Postlayout simulation puts the energy consumption at 2.6 pJ/bit at 4 V and 7.2 pJ/bit at 20 V, with near symmetric rise and fall delays
Excited state non-adiabatic dynamics of pyrrole:A time-resolved photoelectron spectroscopy and quantum dynamics study
The dynamics of pyrrole excited at wavelengths in the range 242-217 nm are studied using a combination of time-resolved photoelectron spectroscopy and wavepacket propagations performed using the multi-configurational time-dependent Hartree method. Excitation close to the origin of pyrrole's electronic spectrum, at 242 and 236 nm, is found to result in an ultrafast decay of the system from the ionization window on a single timescale of less than 20 fs. This behaviour is explained fully by assuming the system to be excited to the A2(\u3c0\u3c3 17) state, in accord with previous experimental and theoretical studies. Excitation at shorter wavelengths has previously been assumed to result predominantly in population of the bright A1(\u3c0\u3c0 17) and B2(\u3c0\u3c0 17) states. We here present time-resolved photoelectron spectra at a pump wavelength of 217 nm alongside detailed quantum dynamics calculations that, together with a recent reinterpretation of pyrrole's electronic spectrum [S. P. Neville and G. A. Worth, J. Chem. Phys. 140, 034317 (2014)], suggest that population of the B1(\u3c0\u3c3 17) state (hitherto assumed to be optically dark) may occur directly when pyrrole is excited at energies in the near UV part of its electronic spectrum. The B1(\u3c0\u3c3 17) state is found to decay on a timescale of less than 20 fs by both N-H dissociation and internal conversion to the A2(\u3c0\u3c3 17) state.Peer reviewed: YesNRC publication: Ye
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