[molpro-user] RS2 gradient error: WRONG E0 FROM CIGAM0I

[Artur Nenov]

Dear Molpro developers,

I have encountered an error while trying an RS2 optimization in Molpro. It 
occurs while Molpro tries to calculate the gradient in a SS-PT2 or MS-MR-PT2 
step. 
The error output reads 

WRONG E0 FROM CIGAM0I: -34.84434593   -34.84434594 (exemplary numbers)

and comes after the RESULTS FOR STATE list. The values given after the error 
message give the Zeroth-order valence energy, which is calculated once in the 
begining of the RS2 routine (without level shift) and then recalculated when a 
gradient calculation is requested (this time adding the level shift). The 
error occurs also when no error shift given.
The error can be straced back to the files cigam2ppi.f (SS-PT2) and 
cigam2ppi_ms.f (MS-MR-PT2), where an accuracy of 1.d-9 for the delta of both 
values is needed, otherwise the program aborts. Obviously, in the given case 
the delta is higher. I all situation where I had this error so far both values 
differ by one or two digits in the very last printed decimal place. Further so 
far this error has not occured with smaller molecules like ethylene or 
cyclohexadiene, only with bigger molecules.

My question is whether these small discrepances between the Zeroth-order 
valence energies before and after the PT2 correction are due to some numerical 
inconsistence or there is more to it. 
Can I ignore this error (comment the abortion execution signal in the two files 
above) or reduce the accuracy? Is this error somehow connected to the accuracy 
of the calculated gradient?

Best regards,
Artur Nenov