About fast

FAST is a linear response time dependent density functional program for computing the electronic absorption spectrum of molecular systems. It uses an $O(N^3)$ linear response method based on finite numerical atomic orbitals and deflation of linear dependence in atomic orbital product space. This version is designed to work with data produced by the SIESTA DFT code. The code produces as principal output a numerical absorption spectrum (complex part of the polarisability, loosely called the polarisability below) and a list of transition energies and oscillator strengths deduced from fitting Lorentzians to the numerical spectrum. Considering the absence of hybrid functionals in SIESTA and that concerning calculation of spectra, generalised gradient Hamiltonians are not usually considered to be notably better than the local density approximation, the present release of FAST works only with LDA, which despite its limitations, has provided useful results on the systems to which the present authors have applied it.

This Reference Manual contains descriptions of all the input, output and execution features of FAST, but is not intended as a tutorial introduction to the program. Potential users should consult the papers listed below for details of the methods employed.

The code is written in Fortran 95 with dynamic memory allocation. It uses routines from the BLAS, LAPACK and FFTW package (see http://www.fftw.org/http://www.fftw.org/) for all FFT routines. It may be compiled for serial or parallel execution (under OpenMP, MPI and MPI+OpenMP). Owing to licence related reasons, users should first obtain and install either the SIESTA code or the standalone SIESTA XC exchange-correlation library ( http://www.icmab.es/siestahttp://www.icmab.es/siesta ) . The present code includes two pieces of software under licence :

• Routines for numerical quadrature on the sphere, distributed through CCL www.ccl.nethttp://www.ccl.net/.
  V.I. Lebedev, and D.N. Laikov ``A quadrature formula for the sphere of the 131st algebraic order of accuracy'' Doklady Mathematics, Vol. 59, No. 3, 1999, pp. 477-481.
• A GMRES solver available at www.cerfacs/algor/Softshttp://www.cerfacs/algor/Softs.

  V. Frayssé, L. Giraud, S. Gratton, and J. Langou, A set of GMRES routines for real and complex arithmetics on high performance computers, CERFACS Technical Report TR/PA/03/3, 2003.