how to build lat.in file?

Alloy Theoretic Automated Toolkit (ATAT)
1

Dear sir,
I want to study a solid solution with orthorhombic structure,which has no symmetry, through the Cluster Expansion code,now hou do I build the lat.in file ? Should I just writ the lattice as its POSCAR (VASP),or choose one from the given structures,such as bcc,fcc,hcp ? Besides,I wonder is there any relationship between the structure in lat.in and the structures of ground states ? In other words,how to choose a structure as the input file befor we calculate the ground states through Cluster Expansion?

2

Can we use cluster expansion for interstitial solid solution? and interstitial compound?

3

These are very fundamental open-ended questions! You might want to look at standard alloy theory references:
title="Order and Phase Stability in Alloys",
author="F. Ducastelle",
publisher="Elsevier Science",
address="New York",
year="1991"

Here are some more ATAT-focused refs:
https://arxiv.org/abs/cond-mat/0201511
https://arxiv.org/abs/cond-mat/0212159

Let me try to answer some of the more specific questions:

  1. even though a solid solution has no symmetry, the lat.in would list all species that can occupy each site. The symmetry of lat.in is much higher: it would be as if you ignore the distinction between the species residing on the same sublattice.
  2. For interstitials, you can create a sublattice that holds either some species or vacancies (denoted Vac).