[Pw_forum] Spin-Polarized NEB

[Giuseppe Mattioli]

Dear Victor
I see two levels at which your question can be answered.
At a technical level, the pw.x code (the "engine" of neb.x) starts from scratch the scf cycle of every image. If all your images converge the first scf 
iteration to a non-magnetic state then a) they will propagate the non-magnetic state to the following neb steps and b) the transition state might 
really be non-magnetic. You can force a paramagnetic ground state (tot_magnetization =/ 0) to have a paramagnetic transition state. However, note that 
at a "quantum chemistry" level, if you are right then your system jumps somewhere between a 2S+1=1 to a 2S+1=3 state (and then revert to a 2S+1=1 
state in the end? Sounds strange but not unheard of, e.g. the rotation of ethylene around the double bond), and this cannot be mapped along a single 
PES. You could calculate the fully diamagnetic and fully paramamagnetic paths (i.e., including initial and final configurations, by constraining the 
tot_magnetization value) and then see if the paramagnetic curve goes below the diamagnetic curve around the transition state. Be aware of the fact 
that if your transition state is intrinsically a multireference state (e.g., a diradical like twisted ethylene, which cannot be described by a single 
determinant), then DFT-based numbers could be poor numbers...
HTH
Giuseppe

On Monday, February 08, 2016 06:26:29 AM Vic Bermudez wrote:
> Hello,
> 
> 	I'm doing a NEB calculation for a reaction where I suspect that the
> transition state might be paramagnetic. In the "engine_input" section I set
> 'nspin=2', and I set a non-zero starting magnetization for one of the atom
> types. In the first SCF cycle of the first NEB iteration the absolute
> magnetization is initially non-zero and converges to a zero absolute
> magnetization. The total magnetization remains zero at all times. This
> applies to all 15 of the NEB images.
> 	However, in successive NEB iterations the SCF starts right from the
> beginning with zero absolute and total magnetizations and remains at zero
> for both quantities right up to SCF convergence.
> 	Here's my question. Given all this, am I really doing a spin-polarized
> calculation, or is my calculation actually spin-restricted. If the latter,
> what should I be doing to get a spin-unrestricted NEB run ?
> 
> Thank you in advance for your help.
> 
> This is the relevant part of the input. Here I'm restarting a calculation
> that was spin-restricted and appeared to be getting somewhat close to NEB
> convergence
> 
> BEGIN
> BEGIN_PATH_INPUT
> &PATH
> restart_mode='restart',
> string_method='neb',
> ds=1.D0,
> opt_scheme='broyden',
> nstep_path=4000,
> num_of_images=15,
> k_max=0.6D0,
> k_min=0.1D0,
> first_last_opt=.TRUE.,
> use_freezing=.TRUE.,
> CI_scheme='auto'
> /
> END_PATH_INPUT
> 
> BEGIN_ENGINE_INPUT
> &CONTROL
> title='NEB Run',
> ! restart_mode='from_scratch',
> pseudo_dir='/lustre/cmf/scratch/b/bermudez/QE_PP/',
> ! outdir='/lustre/cmf/scratch/b/bermudez/NEB-SO2_2/',
> etot_conv_thr=1.0D-4,
> forc_conv_thr=1.0D-3,
> verbosity='default'
> /
> 
> &SYSTEM
> ibrav=8,
> a=14.5898, b=14.7815, c=35.00,
> nat=138,
> ntyp=4,
> ecutwfc=40.0,
> ecutrho=400.0,
> nspin=2,
> starting_magnetization(1)=0.0,
> starting_magnetization(2)=0.0,
> starting_magnetization(3)=0.0,
> starting_magnetization(4)=0.3333,
> london=.true.,
> occupations='smearing',
> smearing='gaussian',
> degauss=0.002,
> nosym=.TRUE.
> /
> 
> &ELECTRONS
> electron_maxstep=200,
> conv_thr=3.0D-9,
> mixing_beta=0.7D0,
> mixing_ndim=8,
> mixing_mode='local-TF'
> /
> 
> ATOMIC_SPECIES
> Zr 91.22   Zr.pbe-spn-kjpaw_psl.0.2.3.UPF
> O  15.9994  O.pbe-n-kjpaw_psl.0.1.UPF
> H   1.0079  H.pbe-kjpaw_psl.0.1.UPF
> S  32.06    S.pbe-n-kjpaw_psl.0.1.UPF
> 
> 
> Victor M. Bermudez
> E-mail: bermudez at alum.mit.edu
> 
> 
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   Giuseppe Mattioli                            
   CNR - ISTITUTO DI STRUTTURA DELLA MATERIA   
   v. Salaria Km 29,300 - C.P. 10                
   I 00015 - Monterotondo Stazione (RM), Italy    
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