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Acording to Dynare Manual, the variable oo_.var is constructed as follows: "After a run of stoch_simul, contains the variance-covariance of the endogenous variables. Contains theoretical variance if the periods option is not present (or an approximation thereof for order=2), and empirical variance otherwise."
I was wondering what is the approximation that Dynare implementes if periods=0 and order=2 in stoch_simul. Could someone please describe how is this is calculated, or simple provide a reference that I can read about it? Thanks!