The basis set generation in CP2K

[ningz... at gmail.com]

Dear all CP2K experts,

   I need to generate a basis set for U. And I see the example of generating gaussian exponent by ATOM code for U:
&GLOBAL
  PROGRAM_NAME ATOM
&END GLOBAL
&ATOM
  ELEMENT U
  ELECTRON_CONFIGURATION [Rn] 5f3 6d1 7s2

  &METHOD
     METHOD_TYPE  KOHN-SHAM
     RELATIVISTIC DKH(3)
     &XC
       &XC_FUNCTIONAL PBE
       &END XC_FUNCTIONAL
     &END XC
  &END METHOD

  &OPTIMIZATION
    MAX_ITER  12
    DAMPING   0.8
    EPS_DIIS 100.
    N_DIIS 6
  &END OPTIMIZATION

  &AE_BASIS
     BASIS_TYPE  GEOMETRICAL_GTO
     NUM_GTO  16 14 12 12
     GEO_START_VALUE   0.2
     GEOMETRICAL_FACTOR 3.2
     START_INDEX  0 0 2 2
  &END AE_BASIS

&END ATOM

    But I have a question of NUM_GTO keyword: why we choose 16 GTO function for s, and 14 for p, 12 for d and f? What is the meaning of START_INDEX.
    Thank you!

    Zhi

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