When calculating the CI between S0 and T1, the following error occurs

Questions
#1

When calculating the CI between S0 and T1, the following error occurs：

We are using <S**2> to determine which singlet excited state is the target state.
Determine which singlet state to optimize in UCIS or Spin-Flip UCIS
A threshold of <S* *2> = 1.00 is used to identify singlet states
Excited state 1, <S^2> = 1.2039: is it a singlet? No.
Excited state 2, <S^2> = 1.6560: is it a singlet? No.
Excited state 3, <S^2> = 1.0600: is it a singlet? No.
Excited state 4, <S^2> = 1.1126: is it a singlet? No.
Excited state 5, <S^2> = 1.1399: is it a singlet? No.
Excited state 6, <S^2> = 1.0561: is it a singlet? No.
Excited state 7, <S^2> = 1.2408: is it a singlet? No.
Excited state 8, <S^2> = 1.9744: is it a singlet? No.
Excited state 9, <S^2> = 1.3155: is it a singlet? No.
Excited state 10, <S^2> = 1.4140: is it a singlet? No.

Q-Chem fatal error occurred in module drvman/cis_choose_state.C, line 105:

Target singlet state not found for UCIS or SF-UCIS gradient!!!

Please submit a crash report at Q-Chem Crash Reporter

My input file is as follows:
$molecule
0 3
O 6.3899880950 -0.0134726842 -0.3098432908
C -0.6913400286 -0.5114708694 0.5242187733
C -0.6158037155 -1.8740083651 0.7378634012
H 0.3582098964 -2.3339559001 0.8730450964
C -1.8090161361 -2.5963157179 0.6073248569
H -1.8448728426 -3.6447755531 0.8897728237
C -3.0099699101 -1.9713637958 0.1609001926
C -4.2396256346 -2.6676776446 0.1547643240
H -4.2617973226 -3.6919866155 0.5067556769
C -5.4554322992 -2.0308012426 -0.1840541086
H -6.4012960890 -2.5633044726 -0.1172877134
C -5.4945870966 -0.6880257058 -0.5149088233
H -6.4426142624 -0.1961103244 -0.7229736131
C -4.2944422250 0.0854491667 -0.4464386583
C -4.2774366822 1.4830324345 -0.6376096827
H -5.1755487197 2.0366473351 -0.9463009354
C -3.0842947959 2.1955088969 -0.4397486891
H -3.0855702942 3.2721419636 -0.5390122338
C -1.9126031796 1.5737557935 0.0466256589
C -1.8904066056 0.1570848149 0.1711727828
C -3.0674131933 -0.5807905784 -0.1154124031
C -0.8030270803 2.3846435185 0.6696284667
H -1.0779002807 2.8179577000 1.6393498013
C 0.5335636225 2.4038100256 0.3970519356
H 1.0337230812 2.9761683549 1.1734905885
C 1.2460978772 1.8696967878 -0.7364259003
C 2.2629953997 0.9901002919 -0.2478097433
C 3.3197894711 1.7236133445 0.4103849813
H 3.0918113715 2.6581415333 0.8775034834
C 4.6517420334 1.3510936260 0.3445487906
H 5.4749773101 1.9724894826 0.7222742339
C 5.0465992589 0.1782903817 -0.2965151359
C 4.0309810389 -0.7244883399 -0.7355489630
H 4.3304764212 -1.6865350160 -1.1813591091
C 2.7051091932 -0.3262463608 -0.7220325582
H 1.9615996995 -1.0774932300 -1.0204865147
C 6.9703570328 -1.2598620503 -0.6655653336
H 6.4893822296 -2.1020656410 -0.1505067850
H 7.9990092695 -1.1385955089 -0.3204626510
H 6.9455570915 -1.4375358352 -1.7502230226
$end

$rem
JOBTYPE opt
MECP_OPT true
MECP_METHODS branching_plane
METHOD b3lyp
BASIS 6-31G(d)
SPIN_FLIP true
UNRESTRICTED true
CIS_N_ROOTS 10
MECP_STATE1 [0,1]
MECP_STATE2 [1,1]
CIS_S2_THRESH 100
sym_ignore true
symmetry off
set_iter = 300
geom_opt_max_cycles = 300
max_scf_cycles = 300
max_cis_cycles = 300
SOLVENT_METHOD smd
GEOM_OPT_COORDS 0
scf_algorithm = gdm
$end

$smx
solvent OTHER
$end

Thank you for your guidance！

#2

Due to significant spin contamination, the code is unable to find any state that it considers to be a singlet, therefore cannot optimize on a singlet state. The threshold for this determination is set at <S^2> = 1, as indicated, which is halfway between singlet and triplet value for spin-pure states.

However, does the manual say that MECP optimization is enabled between singlet and triplet states (as compared to states of the same spin symmetry)? If so, that is news to me.

#3

Thanks for your reply. I looked up the manual and didn’t see the MECP optimization between singlet and triplet states, Perhaps I set MECP_STATE2 [1,1] to represent the T1 state is wrong. I will rethink this question.