When constarin correlations to zero?

[Georg II]

Constraining correlations to zero changes the difficulty and ability parameter estimates slightly and the deviance substantially. When dimensions are free to correlate fit is better (deviance is smaller) compared to when correlations are set to zero.

When and under what conditions should we constrain correlations to zero?

Thanks

[ConQuest]

In a between item context constraining them to zero is just like doing separate runs, so I can't think of many contexts in which you'd want to do it.