convergence problem in scf

[umklapp]

Dear all,
I am doing LDA+U calculation for total energy and electronic structure of Ti-doped corundum Cr2O3(hexagonal cell) using tetrahedron method. I perform the self-consistent calculation using a gamma-centered K mesh and the following INCAR :
ISTART = 0
ICHARG = 2
NPAR = 1
ISYM = 0
IALGO = 38
LDIAG = T
LREAL = A
NELM = 40
NELMDL = 0
EDIFF = 1E-5
PREC = Accurate
ENCUT = 520
ADDGRID = T
LMAXMIX = 4
ISMEAR = -5
ISPIN = 2
MAGMOM = 5 -5 5 -5 5 -5 5 -5 5 -5 5 -5 18*0
VOSKOWN = 1
LNONCOLLINEAR = F
LSORBIT = F.
SAXIS = 0 0 1
NSW = 0
IBRION = -1
LWAVE = T
LCHARG = T
LDAU = T
LDAUTYPE = 1
LDAUL =2 2 -1
LDAUU =4.0 4.0 0.0
LDAUJ =0.58 0.58 0.0
LDAUPRINT =2
The converged energy I get is :
1 F= -.26323234E+03 E0= -.26323234E+03 d E =0.000000E+00 mag= -1.9996
Using the converged charge density and wave function in the self-consistent calc, I run more scf iterations. I only modify the INCAR file slightly, ISTART = 1 ICHARG = 1. I thought it would take only a few iterations to converge to the same energy. However, although it converges to a similar energy, the calculation takes 53 iterations. I wonder why so many iterations are still needed. The charge density is supposed to converge.
The OSICAR of the 2nd self-consistent calc is as follows:
DAV: 1 -0.262815731213E+03 -0.26282E+03 -0.26949E-01 13728 0.132E+00 0.178E+00
DAV: 2 -0.263143852861E+03 -0.32812E+00 -0.41780E-01 12688 0.297E+00 0.146E+00
……
DAV: 52 -0.263232397806E+03 0.44054E-05 -0.14918E-04 11848 0.462E-02 0.757E-03
DAV: 53 -0.263232400277E+03 -0.24713E-05 -0.56626E-05 8536 0.322E-02
1 F= -.26323240E+03 E0= -.26323240E+03 d E =0.000000E+00 mag= -2.0000
Could anybody explain to me why the energy of 1st iteration in the last calc is so much different from the previous total energy?
Thanks very much!

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