open-standards/wai/wai-quantum/examples/dmrg_chain.rs

//! Ground-state DMRG on a spin chain, against the exact answer where one
//! exists.
//!
//! ```text
//! cargo run --release --example dmrg_chain --features quantum_dmrg -- ising|heisenberg N BOND SWEEPS [h]
//! ```
use wai_quantum::quantum_dmrg::{dmrg, exact_ground_energy, ising_exact_energy, Chain, DmrgConfig};

fn main() {
    let a: Vec<String> = std::env::args().collect();
    let n: usize = a[2].parse().unwrap();
    let bond: usize = a[3].parse().unwrap();
    let sweeps: usize = a[4].parse().unwrap();
    let field: f64 = a.get(5).map_or(1.0, |x| x.parse().unwrap());
    let (chain, exact) = match a[1].as_str() {
        "ising" => (Chain::ising(n, 1.0, field), Some(ising_exact_energy(n, 1.0, field))),
        "heisenberg" => {
            let c = Chain::heisenberg(n, 1.0, 1.0, 0.0);
            let e = if n <= 20 { exact_ground_energy(&c) } else { None };
            (c, e)
        }
        other => panic!("{other}"),
    };
    let t0 = std::time::Instant::now();
    let r = dmrg(&chain, &DmrgConfig { max_bond: bond, sweeps, ..DmrgConfig::default() });
    let dt = t0.elapsed().as_secs_f64();
    let mid = r.entropies[n / 2 - 1];
    print!("{} n={n} bond={bond} sweeps={sweeps}  E={:.12}  per site {:.12}  discarded {:.1e}  S(mid) {:.4}", a[1], r.energy, r.energy / n as f64, r.discarded, mid);
    if let Some(e) = exact {
        print!("  exact {e:.12}  error/site {:.1e}", (r.energy - e) / n as f64);
    }
    println!("  {dt:.1}s");
    println!("sweeps: {:?}", r.sweep_energies.iter().map(|e| format!("{e:.10}")).collect::<Vec<_>>());
}