ATC: fix_modify AtC fields

# fix_modify AtC fields

## syntax

fix_modify AtC fields <all | none>
fix_modify AtC fields <add | delete> <list_of_fields>

- all | none (keyword) = output all or no fields
- add | delete (keyword) = add or delete the listed output fields
- fields (keyword) =
  density : mass per unit volume
  displacement : displacement vector
  momentum : momentum per unit volume
  velocity : defined by momentum divided by density
  projected_velocity : simple kernel estimation of atomic velocities
  temperature : temperature derived from the relative atomic kinetic energy (as done by )
  kinetic_temperature : temperature derived from the full kinetic energy
  number_density : simple kernel estimation of number of atoms per unit volume
  stress : Cauchy stress tensor for eulerian analysis (atom_element_map), or 1st Piola-Kirchhoff stress tensor for lagrangian analysis
  transformed_stress : 1st Piola-Kirchhoff stress tensor for eulerian analysis (atom_element_map), or Cauchy stress tensor for lagrangian analysis
  heat_flux : spatial heat flux vector for eulerian, or referential heat flux vector for lagrangian
  potential_energy : potential energy per unit volume
  kinetic_energy : kinetic energy per unit volume
  thermal_energy : thermal energy (kinetic energy - continuum kinetic energy) per unit volume
  internal_energy : total internal energy (potential + thermal) per unit volume
  energy : total energy (potential + kinetic) per unit volume
  number_density : number of atoms per unit volume
  eshelby_stress: configurational stress (energy-momentum) tensor defined by Eshelby [References: Philos. Trans. Royal Soc. London A, Math. Phys. Sci., Vol. 244, No. 877 (1951) pp. 87-112; J. Elasticity, Vol. 5, Nos. 3-4 (1975) pp. 321-335]
  vacancy_concentration: volume fraction of vacancy content
  type_concentration: volume fraction of a specific atom type

## examples

`fix_modify AtC fields add velocity temperature`

## description

Allows modification of the fields calculated and output by the transfer class. The commands are cumulative, e.g.
`fix_modify AtC fields none`
followed by
`fix_modify AtC fields add velocity temperature`
will only output the velocity and temperature fields.

## restrictions

Must be used with the hardy/field type of AtC fix, see fix atc command. Currently, the stress and heat flux formulas are only correct for central force potentials, e.g. Lennard-Jones and EAM but not Stillinger-Weber.

## related

See fix_modify AtC gradients , fix_modify AtC rates and fix_modify AtC computes

## default

By default, no fields are output