Excited states

src/fciqmc.jl

@@ -188,4 +188,20 @@ end
 function advance!(algorithm, report, state::ReplicaState, replica::SpectralState{1})
     return advance!(algorithm, report, state, only(replica.single_states))
 end
-# TODO: add advance! for SpectralState{N} where N > 1
+
+function advance!(algorithm, report, state::ReplicaState, replica::SpectralState{N, <:Any, GramSchmidt{N}}) where {N}
+    proceed = true
+    if state.step[] % replica.spectral_strategy.orthogonalisation_interval == 0
+        for i in 1:N
+            for j in 1:i-1
+                u = replica[i].v
+                v = replica[j].v
+                add!(u, v, -dot(u, v) / dot(v, v))
+            end
+        end
+    end
+    for i in 1:N
+        proceed &= advance!(algorithm, report, state, replica[i])
+    end
+    return proceed
+end

src/strategies_and_params/spectralstrategy.jl

@@ -18,11 +18,12 @@ num_spectral_states(::SpectralStrategy{S}) where {S} = S
     GramSchmidt{S} <: SpectralStrategy{S}
 
 Use the Gram-Schmidt procedure to orthogonalize the excited states. A total of `S` spectral
-states are used in the simulation.
+states are used in the simulation, and they are orthogonalised every `orthogonalisation_interval` steps.
 """
-struct GramSchmidt{S} <: SpectralStrategy{S} end
+struct GramSchmidt{S} <: SpectralStrategy{S}
+    orthogonalization_interval::Int
+end
 
-function GramSchmidt(num_spectral_states = 1)
-    num_spectral_states > 1 && throw(ArgumentError("num_spectral_states > 1 is currently not supported."))
-    GramSchmidt{num_spectral_states}()
+function GramSchmidt(S = 1; orthogonalization_interval = 1)
+    return GramSchmidt{S}(orthogonalization_interval)
 end