= Change the amount of HF exchange in functionals =

Currently, only possible for BLYP series. The only modification in SCF section is
{{{
$scf
...
DFT
# new functional with cX*HF+(1-cX)*B88
VBLYP
hfexchange
0.0
...
$end
}}}

Results for ground and first-two triplet excited states of CH2O:

||HF c_X ||Etot(a.u.) ||w1(eV) ||w2(eV) ||
||0.0 (BLYP)       ||  -114.58954137 ||3.1626 (A2)  || 5.5590 (A1) ||
|| 0.25              ||  -114.56629144 || 3.2367 (A2) || 5.3229 (A1) ||
|| 0.5 (BHHLYP) ||  -114.54640539 || 3.3246 (A2) || 4.7647 (A1) ||
|| 0.75              ||   -114.52966660 || 3.4278 (A2) || 4.1320 (A1) ||
|| 1.0 (HF-LYP) ||  -114.51588588 || 3.3983 (A1) || 3.5477 (A2) ||
|| ||
|| HF ||  -113.97616771 || 2.1812 || 3.4426 ||

'''Note that when c_X=1, it is not HF but HF-LYP.'''

{{attachment:ch2o.png||width=600,align="middle"}} 

Full input:
{{{

$COMPASS
Title
 ch2o 
Basis
 aug-cc-pvtz
Geometry
C        0.000000    0.000000   -0.526806
O        0.000000    0.000000    0.677133
H        0.000000    0.945218   -1.128115
H        0.000000   -0.945218   -1.128115
End geometry
$END

$xuanyuan
scalar
heff
3
$end

$scf
RKS
DFT
VBLYP
hfexchange
0.75
charge
0
spin
1
THRESHCONV
1.d-12 1.d-10
OPTSCR
1
$end

$traint
tddft
orbi
hforb
$end

$tddft
imethod
1
isf
1
idiag
1
iexit
2
itda
0
istore
1
$end

}}}