# Tutorial:Optical spectra from time-propagation

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In this tutorial we will learn how to obtain the absorption spectrum of a molecule from the explicit solution of the time-dependent Kohn-Sham equations. We chose as a test case methane (CH_{4}).

## Ground-state

Before starting out time-dependent simulations, we need to obtain the ground-state of the system. For this we use basically the same ** inp** file as in a previous tutorial

CalculationMode = gs Units = eV_angstrom radius = 4 spacing = 0.175 CH = 1.2 %Coordinates "C" | 0 | 0 | 0 | no "H" | CH/sqrt(3) | CH/sqrt(3) | CH/sqrt(3) | no "H" | -CH/sqrt(3) |-CH/sqrt(3) | CH/sqrt(3) | no "H" | CH/sqrt(3) |-CH/sqrt(3) | -CH/sqrt(3) | no "H" | -CH/sqrt(3) | CH/sqrt(3) | -CH/sqrt(3) | no %

After running Octopus, we will have the Kohn-Sham wave-functions of the ground-state in the directory ** restart/gs**.