MCTDHX forum
MCTDHX Usage => Main Program Usage => Topic started by: chrisapo on February 10, 2020, 10:20:14 AM

Hello. Thank you for your useful answers. I came up with a few others :)
I noticed that the default behavior of MCTDHX's last version, when the GUESS='HAND' option is set, is to start from orbitals that are random functions. Is there a particular reason for this? Does the program work better with this random starting point?
Also, from this I deduce that the starting orbitals do not need to be orthogonal to each other. Am I correct?

Hi chrisapo!
Questions are always appreciated! Sorry for the delayed reply.
Regarding: the random choice of initial conditions
The algorithm to compute ground states via imaginary time propagation of the MCTDHX working equations is generally very insensitive to the choice of initial conditions.
However, there are some pitfalls. For instance, when initial state (determined by coefficients and orbitals) has exactly zero overlap with the ground state that is sought.
The motivation for the random choice of initial conditions is to guarantee that the overlap with any possible state is finite. This random choice typically makes the initial steps of the integration a bit slower.
Regarding orthonormality: the orbitals (working as well as natural) should always be strictly orthonormal. For handgivenorbitals, this is ensured by a GramSchmidt orthonormalization.
During realtime evolution, orthonormality is a property that follows from the projection operator in the equations of motion and the unitarity of the Hamiltonian.
During imaginarytime evolution, the orthonormality is guaranteed by a GramSchmidt orthonormalization at every integration step.
Hope that helps!