Defining the initial state $\vert \Psi \rangle$:

The MCTDH-X wavefunction is a multiconfigurational expansion with time-dependent coefficients $C_{\vec{n}}(t)$ and time-dependent configurations $\vert \vec{n};t\rangle$. In order to fully define the wavefunction,

$$\vert \Psi (t) \rangle = \sum_{\lbrace \vec{n} \rbrace} C_{\vec{n}}(t) \vert \vec{n};t\rangle,$$ (2)

one has to define all coefficients $C_{\vec{n}}$ and all configurations $\vert \vec{n};t\rangle$. In the case of a fully user-defined guess, i.e., GUESS=`HAND' in the input file, the wavefunction is supplied via Fortran routines. The coefficients are defined in the file source/ini_guess_pot/Get_Initial_Coefficients.F and the orbitals from which the configurations are built are defined in the file Get_Initial_Orbitals.F in the directory source/ini_guess_pot/. Both contain a single Fortran subroutine, that assigns the corresponding array (for a documentation of the routines, please see the html documentation). For a relaxation, i.e., the calculation of a certain eigenstate of the many-body Hamiltonian $\hat{H}$, usually it is a good choice to start from a Gross-Pitaevskii, i.e., single-configurational state with $\vert \Psi \rangle = \vert N,0,0,... \rangle$. For a propagation there is also the possibility to restart the computation from a previous one by specifying GUESS=`BINR' in the input file.