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I calculate the phonon spectrum of bcc lithium using PHONON code together with
vasp. however I get a large imaginary frequency at 0 Gpa, which should be positive accordingg to others calculation.my pesudo potienials is PAW-lda, and I have search the forum and read anything about imaginary frequency,and have paid attention to the ENCUT ,POTIM, the BOX length. and the system also has been relaxed enough, would anyone tell me where is wrong? the following is the INCAR KPONTS, is it related to the pseudo potental or the ISMEAR=1,for metal ,when calculate the frequency ,what to use for
ISMEAR ?
any help will be appreciated. thank you very much.
ISTART = 0
IBRION =2
ISIF =2
PREC = High
EDIFF = 1.0D-07
POITM = 0.015
ENCUT = 250
ISMEAR = 1
SIGMA = 0.2
NSW = 1
ADDGRID = .TRUE.
LWAVE=.FASLE.
LCHARGE=.FALSE.

Your input seems very reasonable to me. Have you checked whether you are in the PAW-LDA equilibrium geometry of Li (external pressure on the cell should be less than 1 kB, the remaining forces on the ions should be about 0.01 eV/Ã… or less )? If so please, first of all check whether the error is related to VASP or to PHONON. (do test run of VASP using IBRION=5 and have a look at the phonon frequencies given by vasp)