Hi,

I am analyzing several waves of longitudinal dataset to see whether there are significant interaction between genetic factor (pgsgeneral) and trauma experience(dichokid, 0=no, 1=yes) on cognitive measures (crystal), after controlling for covariates (variables listed after the syntax global control) using xtmixed.
Syntax that I am using is as below.

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global control "rfemale rage rage_sq rempl rret rsd rsingle rfair rgood rvgood rexcel rhibp rdiab rc
> ancr rlung rheart rstrok rcesd rpsych rarthr i.rsmokcat i.rbmicat i.rlight lnrdrink ihshwealth i.edu
> cat hnoppl chld rhins lnhinc PCA PCB PCC PCD PCE"

. xtmixed crystal c.pgsgeneral##i.dichokid $control || hhidpn:rage if sample_woedu_1==1, covariance(un
> structured) mle

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However, after running this syntax, I encountered the message that Hessian is not negative semidefinite. Please see below. However, when I used different outcome measures other than crystal, it does not show the error message. Crystal is such an important outcome measure, so I am trying to find a way to solve this error.
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Performing EM optimization:

Performing gradient-based optimization:

Iteration 0: log likelihood = -23195.549
Iteration 1: log likelihood = -22974.72 (not concave)
Iteration 2: log likelihood = -22951.83 (not concave)
Iteration 3: log likelihood = -22942.059 (not concave)
Iteration 4: log likelihood = -22937.871
Iteration 5: log likelihood = -22925.872
Iteration 6: log likelihood = -22921.494
Iteration 7: log likelihood = -22921.452
Iteration 8: log likelihood = -22921.452
Iteration 9: log likelihood = -22921.452
Hessian is not negative semidefinite
r(430);

end of do-file

r(430);
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In this case, what does this error message mean? I was trying to find the answer by googling this stata forum and several literature, but I have no clue yet.
I am wondering if there would be anyone who would have some clue how I can address this issue. Thank you very much in advance for any of your input or experience.