Re: About changing charge/multiplicity during QM-MM

From: Gerard Rowe (GerardR_at_usca.edu)
Date: Fri Apr 19 2019 - 07:46:31 CDT

Hi Francesco,

That BS solution you converged to seems like it could be a saddle point in the SCF process rather than a true ground state. I think one easy way to confirm that this is a real BS state would be to flip the spin of the other iron atom. Since binuclear nonheme iron active sites are reasonably symmetric, you should converge to the same solution.

When you look at the Mulliken or Loewdin spin population analysis of the BS state, do the iron atoms still have opposite spin polarizations? And are they roughly equal in magnitude?

-Gerard

________________________________
From: Francesco Pietra <chiendarret_at_gmail.com>
Sent: Thursday, April 18, 2019 4:02 PM
To: Gerard Rowe
Cc: NAMD
Subject: Re: namd-l: About changing charge/multiplicity during QM-MM

Hi Gerard:
I have reexamined along BS-B3LYP the intermediate where O2 has become bonded to FE-FE, bridging them. The one that with settings "! B3LYP RIJONX def2/J " gave exchange coupling of 19000 cm-1 and ferromagnetic coupling.

Now, with settings
| 1> ! B3LYP def2-SV(P) [split valence with polarization on heavy atoms only]
| 2> %scf MaxIter 10000
| 3>
| 4> Flipspin 77
| 5> FinalMs 0
| 6> end
| 7>
| 8> *xyz 0 11

Convergence Tolerance:
 Convergence Check Mode ConvCheckMode .... Total+1el-Energy
 Convergence forced ConvForced .... 1
 Energy Change TolE .... 1.000e-06 Eh
 1-El. energy change .... 1.000e-03 Eh
 DIIS Error TolErr .... 1.000e-06

HIGH SPIN SCF CONVERGED AFTER 233 CYCLES
...........................................................................................

FLIP-SPIN BROKEN SYMMETRY

 Flipping spin on atom Fe 77
Before flipping the spin we have:
Multiplicity ... 11
Number of unpaired electrons ... 10
Number of spin-up electrons ... 224
Number of spin-down electrons ... 214
HOMO spin-up ... 223
HOMO spin-down ... 213
After flipping the spin we have:
Multiplicity ... 1
Number of unpaired electrons ... 0
Number of spin-up electrons ... 219
Number of spin-down electrons ... 219
HOMO spin-up ... 218
HOMO spin-down ... 218

                        *
               *BROKEN SYMM SCF CONVERGED AFTER 293 CYCLES

  Last Energy change ... -5.7831e-07 Tolerance : 1.0000e-06
  Last MAX-Density change ... 6.1320e-04 Tolerance : 1.0000e-05
  Last RMS-Density change ... 1.1186e-06 Tolerance : 1.0000e-06
  Last DIIS Error ... 2.7946e-04 Tolerance : 1.0000e-06

BROKEN SYMMETRY MAGNETIC COUPLING ANALYSIS

S(High-Spin) = 5.0
<S**2>(High-Spin) = 30.0214
<S**2>(BrokenSym) = 2.9239
E(High-Spin) = -5203.395729 Eh
E(BrokenSym) = -5203.308495 Eh
E(High-Spin)-E(BrokenSym)= -2.3737 eV -19144.025 cm**-1 (FERROMAGNETIC coupling)

The total simulation took ca 19hr, near at the limit of the allowed 24hr, so that I could not use a higher basis set (or I should ask for >24hr, which is difficult top obtain).

francesco

On Mon, Apr 15, 2019 at 3:06 PM Gerard Rowe <GerardR_at_usca.edu<mailto:GerardR_at_usca.edu>> wrote:
Hi Francesco,

Yes, it is definitely possible. After converging to the BS solution, your NAMD input should specify the multiplicity of the BS state. As far as I'm aware, there is no explicit information in the GBW file that indicates the spin state for Orca to read in, so you always need to let it know what you want. One other note, you should make sure you specify UKS in your input line. Orca will default to a restricted (i.e., not an open-shell BS singlet) wavefunction if it sees a singlet value in the muliplicity line.

-Gerard

________________________________
From: Francesco Pietra <chiendarret_at_gmail.com<mailto:chiendarret_at_gmail.com>>
Sent: Sunday, April 14, 2019 2:31 AM
To: Gerard Rowe; NAMD
Subject: Fwd: namd-l: About changing charge/multiplicity during QM-MM

A BS single point, for the last said system, with settings

! B3LYP def2-TZVP SlowConv
%scf MaxIter 10000

Flipspin 77
FinalMs 0
end

*xyz 0 11

converged HS in 55 SCF iterations, then carried out only 3 BS iterations (in the allowed 24hr).

On restarting with the same input command qmmm.job, the qmmm.gbw file was read, including MO. However, the simulation starts again with very slow HS SCF iterations, i.e., the allowed 24hr would be again too short for BS. From orca manual and web I was unable to find the way to have the simulation starting directly from BS. Is that at all possible?

thanks
francesco

---------- Forwarded message ---------
From: Francesco Pietra <chiendarret_at_gmail.com<mailto:chiendarret_at_gmail.com>>
Date: Fri, Apr 12, 2019 at 5:30 PM
Subject: Re: namd-l: About changing charge/multiplicity during QM-MM
To: Gerard Rowe <GerardR_at_usca.edu<mailto:GerardR_at_usca.edu>>
Cc: namd-l_at_ks.uiuc.edu<mailto:namd-l_at_ks.uiuc.edu> <namd-l_at_ks.uiuc.edu<mailto:namd-l_at_ks.uiuc.edu>>

Yes I noticed the problem, especially the ferromagnetic. Nonetheless I was happy that ice could be broken with B3LYP, although still inadequately. i had already attempted with " ! B3LYP def2-TZVP " but it proved much too slow--_000_DM6PR19MB29873EC6C6CD2D97D29E1098D1270DM6PR19MB2987namp_--

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