I have introduced onsite disorder in Graphene. How will I write the momentum space Hamiltonian for such a system?(System size is 25 x 25 unit cells) If I don't add onsite disorder then we can write the momentum space Hamiltonian(k space) easily. The tight binding Hamiltonian in terms of creation and annihilation operator reads as, $$H = -t \sum_{<i,j>} c_i^{\dagger} c_j + \sum_{i} w_i c_i^{\dagger} c_i$$ where,the first quantity on r.h.s is the nearest neighbor hopping term, t is the nearest neighbor hopping strength, $c_i^\dagger$ is the creation operator, $c_i$ is the annihilation operator. The second quantity is for onsite disorder where, $w_i$ is the randomly generated onsite energy at site $i$.
The important thing here is that the second quantity on r.h.s breaks the translation symmetry.