LU failed in BW-MRCC for 4 roots and 4 references


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For a molecule treated by BW-MRCC using four references, when root =1, the calculation converged at the 15th step; but when root=4, it failed at the 35th step with the following error message
tce_diis: LU decomposition failed
0current input line :       0: 
------------------------------------------------------------------------
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This error has not yet been assigned to a category
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For more information see the NWChem manual at http://nwchemgit.github.io/index.php/NWChem_Documentation
...


The input is like the following

basis spherical
 N library cc-pvdz
H library aug-cc-pvdz
C library cc-pvdz
end

scf
rohf
singlet
thresh 1.0e-10
tol2e 1.0e-10
end

tce
 bwccsd
thresh 1.0e-10
targetsym a’
io ga
end

mrccdata
 root 4
nref 4
...
end

task the energy

From the output

Symmetry of references

Ref. 1 sym:a'
Ref. 2 sym:a'
Ref. 3 sym:a"
Ref. 4 sym:a"


The program persistently requires two orbitals of every atom constituting the molecule plus an additional one to be a reference.