Alternative of TD-DFT: particle-particle TDA (pp-TDA) based properties

The pp-TDA describes N-electron states from a (N-2)-electron reference. The only difference in input is to set imethod=4, itda=1, pprpa=1. The keyword pprpa separate the calculations of N-electron states (=1, pp-TDA) and (N-4)-electron states (=2, hh-TDA). For example, the input for BH is

$COMPASS
Title
 bh
Basis
 cc-pvqz
Geometry
 B 0. 0.  0.
 H 0. 0.  1.232
End geometry
skeleton
group
c(2v)
$END

$xuanyuan
direct
schwarz
$end

$scf
RHF
charge
2
spin
1
THRESHCONV
1.d-10 1.d-8
guess
hcore
$end

$tddft
imethod
4
isf
1
idiag
1
itda
1
pprpa
1
iexit
10
$end

The gradients of single states, and the transition dipole moments and NAC between two states can be computed similar to those for TD-DFT using the resp module.