5,183 research outputs found
Approximability of the Multiple Stack TSP
STSP seeks a pair of pickup and delivery tours in two distinct networks,
where the two tours are related by LIFO contraints. We address here the problem
approximability. We notably establish that asymmetric MaxSTSP and MinSTSP12 are
APX, and propose a heuristic that yields to a 1/2, 3/4 and 3/2 standard
approximation for respectively Max2STSP, Max2STSP12 and Min2STSP12
Electronic excitations from a linear-response range-separated hybrid scheme
We study linear-response time-dependent density-functional theory (DFT) based
on the single-determinant range-separated hybrid (RSH) scheme, i.e. combining a
long-range Hartree-Fock exchange kernel with a short-range DFT
exchange-correlation kernel, for calculating electronic excitation energies of
molecular systems. It is an alternative to the long-range correction (LC)
scheme which has a standard full-range DFT correlation kernel instead of only a
short-range one. We discuss the local-density approximation (LDA) to the
short-range exchange-correlation kernel, and assess the performance of the
linear-response RSH scheme for singlet-singlet and singlet-triplet valence and
Rydberg excitations in the N2, CO, H2CO, C2H4, and C6H6 molecules, and for the
first charge-transfer excitation in the C2H4-C2F4 dimer. The introduction of
long-range HF exchange corrects the underestimation of charge-transfer and
high-lying Rydberg excitation energies obtained with standard (semi)local
density-functional approximations, but also leads to underestimated excitation
energies to low-lying spin-triplet valence states which can be cured by the
Tamm-Dancoff approximation. This work thus suggests that the present
linear-response RSH scheme is a reasonable starting approximation for
describing electronic excitation energies, even before adding an explicit
treatment of long-range correlation
- …