Initial State of the System

Before the actual path integral simulation may be run you have to prepare the initial configuration of the ``quantized'' ionic cores and optimize the electronic wave function in each time slice. Depending on the accuracy you want to achieve (see ref. [197] for how to estimate finite path discretization error) and the computer resources you have available, choose the imaginary time discretization quality using the TROTTER DIMENSION keyword. Typical values for moderate-size molecules are 16, 32 (or more, if you can afford it).

  TROTTER DIMENSION
    16

Note that with 16 time slices you will essentially have to simulate 16 copies of the ``classical'' system and the path integral MD calculation will be at least require 16 times more CPU than a regular Car-Parrinello MD.

Generation of the initial positions of the beads can be steered with the keywords

  INITIALIZATION
  GENERATE REPLICAS
  DEBROGLIE [CENTROID]
    300.D0

For use of the CENTROID suboption see below. The temperature given in Kelvin accounts for the initial dispersion of the quantum particles representing the ionic cores. The lower the temperature the higher dispersion particles have. Be careful here since the dispersion of light particles (like protons) in low temperatures can distort the whole molecule significantly and cause wave function convergence problems. If you do not use RESTART COORDINATES in the &CPMD section the INITIALIZATION and GENERATE REPLICAS options are turned on automatically. Alternatively you can use READ REPLICAS instead of generating them. Now you are ready to prepare the initial state of your system for path integral simulations with an input like the following one:

&CPMD
  PATH INTEGRAL
  OPTIMIZE WAVEFUNCTION
  PCG MINIMIZE
  TIMESTEP
    20
&END

&PIMD
  DEBROGLIE CENTROID
    300.D0
  TROTTER DIMENSION
    16
&END

Obviously the sections &SYSTEM, &DFT and &ATOMS should contain proper input determined earlier.