Units
***************

The default parameters in ``PyChem`` are ::
  
  UnitLength_in_cm                  = 3.085e+21;            # 1.0 kpc */ 
  UnitMass_in_g                     = 1.989e+43;            # 1.0e10 solar masses */ 
  UnitVelocity_in_cm_per_s          = 20725573.785998672;   # 207 km/sec */ 

To change this default, you can pass new values during the initialisatio of ``PyChem``::

  
  params={}
  params['UnitLength_in_cm']          = 3.085e+21          # unit length in kpc
  params['UnitMass_in_g']             = 1.989e+33          # unit mass in solar mass
  params['UnitVelocity_in_cm_per_s']  = 9918338477366.2539 # unit velocity in kpc/yr 

  from PyChem import chemistry
  chemistry.init_chimie("../chemistry.simple.dat",params=params)

The internal units of time will then be in years.
It is possible to retrives these values with the following command::

  chemistry.GetParameters()