I think you have made a small typing error or a search/replace has gone bad.
Your error indicates that you try to do something like
iteration_mixing_parameters = iterationMixingParameters(
algorithm = IterationMixing.Pulay,
diagonal_mixing_parameter = 0.1,
quantity = IterationControlParameters.DensityMatrix, # <--------
history_steps = 6
)
whereas the correct code should read
iteration_mixing_parameters = iterationMixingParameters(
algorithm = IterationMixing.Pulay,
diagonal_mixing_parameter = 0.1,
quantity = IterationMixing.DensityMatrix, # <--------
history_steps = 6
)
I'm guessing a bit, but it seems like this is the problem. If not, please post the whole script (minus the structure, if you don't want to publish it).