Is it a saddle points???

langes · Post by **langes** » Wed May 16, 2007 3:19 am

Dear all.
Now I'm doing a sample dimer run. Yes, the result said the saddle point has been found. But the DIMCAR like this:
Step Force Torque Energy Curvature Angle
1 2.73385 6.42083 -24.46458 0.51900 14.85002
1 2.73385 1.65521 -24.46458 -1.33347 13.88979
1 2.73385 1.16259 -24.46458 -1.77245 7.11482
1 2.73385 0.94642 -24.46458 -1.90382 3.70079
2 1.12340 0.70536 -24.71508 -0.47158 3.78087
3 0.80335 0.91701 -24.79515 0.14902 7.36007
4 0.70593 1.52175 -24.78423 -0.17892 8.53775
4 0.70593 0.39345 -24.78423 -0.36717 5.30432
5 0.93040 0.79134 -24.87029 0.08285 6.14577
6 0.81469 1.52029 -24.85591 -0.30462 10.11215
6 0.81469 0.87274 -24.85591 -0.52902 8.92016
7 0.55798 0.86896 -24.89917 -0.46508 6.82333
8 0.39096 0.59308 -24.95361 -0.45754 8.04966
9 0.51123 0.45989 -24.93681 -0.76110 8.17468
10 0.21532 0.29381 -24.93444 -0.67892 4.05445
11 0.15897 0.21330 -24.93502 -0.75172 2.11039
12 0.09234 0.35706 -24.93270 -0.70992 3.72195
13 0.06173 0.18661 -24.93312 -0.70894 2.74958
14 0.03050 0.20880 -24.93315 -0.73142 1.81750
15 0.02561 0.10924 -24.93314 -0.74337 2.48709
16 0.02360 0.15342 -24.93318 -0.71340 1.30439
17 0.00864 0.07168 -24.93327 -0.72663 1.71032
18 0.01118 0.04765 -24.93330 -0.73179 0.70912
19 0.00631 0.04577 -24.93331 -0.72967 0.45940
20 0.00320 0.03779 -24.93331 -0.73214 0.42693
21 0.00275 0.03491 -24.93331 -0.73412 0.47660
22 0.00206 0.02057 -24.93331 -0.73008 0.31008
That means the lowest energy points is saddle points. Of course,it's wrong. But when I check the position found, it's maybe correct. So what's the matter with it? Please guide me where I did the mistake?

I will post my parameter:
INCAR:
SYSTEM = H on Mg-0001-223 Dimer Run
IBRION=3
POTIM=0.0
ISYM=0
NSW=600
EDIFF=1E-7
EDIFFG=-0.001
PREC=Norm
LREAL=Auto
LWAVE=.FALSE.
LCHARG=.FALSE.

# DIMER PARAMETERS
ICHAIN=2
DdR=0.05
DRotMax=4
DFNMin=0.01
DFNMax=1.0

# OPTIMIZER PARAMETERS
IOPT=2

POSCAR-IS:
Mg H
3.209400000000000
1.7320000000000000 -1.000000000000000 0.0000000000000000
0.0000000000000000 2.000000000000000 0.0000000000000000
0.0000000000000000 0.0000000000000000 7.404530441827132797407615130554
16 1
Selective dynamics
Direct
0.16667 0.33333 0.00000 F F F
0.16667 0.33333 0.21926 F F F
0.16667 0.83333 0.00000 F F F
0.16667 0.83333 0.21926 F F F
0.66667 0.33333 0.00000 F F F
0.66667 0.33333 0.21926 F F F
0.66667 0.83333 0.00000 F F F
0.66667 0.83333 0.21926 F F F
0.33333 0.16667 0.10963 F F F
0.33333 0.16667 0.32889 T T T
0.33333 0.66667 0.10963 F F F
0.33333 0.66667 0.32889 T T T
0.83333 0.16667 0.10963 F F F
0.83333 0.16667 0.32889 T T T
0.83333 0.66667 0.10963 F F F
0.83333 0.66667 0.32889 T T T
0.53000 0.45000 0.32889 T T T

POSCAR-FS:
Mg H
3.209400000000000
1.7320000000000000 -1.000000000000000 0.0000000000000000
0.0000000000000000 2.000000000000000 0.0000000000000000
0.0000000000000000 0.0000000000000000 7.404530441827132797407615130554
16 1
Selective dynamics
Direct
0.16667 0.33333 0.00000 F F F
0.16667 0.33333 0.21926 F F F
0.16667 0.83333 0.00000 F F F
0.16667 0.83333 0.21926 F F F
0.66667 0.33333 0.00000 F F F
0.66667 0.33333 0.21926 F F F
0.66667 0.83333 0.00000 F F F
0.66667 0.83333 0.21926 F F F
0.33333 0.16667 0.10963 F F F
0.33333 0.16667 0.32889 T T T
0.33333 0.66667 0.10963 F F F
0.33333 0.66667 0.32889 T T T
0.83333 0.16667 0.10963 F F F
0.83333 0.16667 0.32889 T T T
0.83333 0.66667 0.10963 F F F
0.83333 0.66667 0.32889 T T T
0.57000 0.35000 0.35000 T T T

Thanks a lot!

andri · Post by **andri** » Wed May 16, 2007 8:31 pm

It looks like a saddle point to me, i.e.:

1) Force is low (and getting smaller)
2) Torque is vanishing.
3) Curvature is negative.

So, smells like a saddle point!

langes · Post by **langes** » Thu May 17, 2007 12:37 pm

[quote="andri"]It looks like a saddle point to me, i.e.:

1) Force is low (and getting smaller)
2) Torque is vanishing.
3) Curvature is negative.

So, smells like a saddle point![/quote]

Thank you, andri!

I think saddle point is the highest energy points in the MEP, though here it can be confirmed with mathematical estimate formula! Can you tell me what's the matter? Is it a bad initio configuration?

andri · Post by **andri** » Thu May 17, 2007 5:28 pm

I am a little bit confused as to why you think your configuration is a minimum. According to your DIMCAR, your initial configuration starts out slightly too high in energy, which is perfectly reasonable. As the calculation progresses, the energy is lowered as the system approaches the saddle point. But the bottom line is that if you have a configuration with zero force (first derivative of the potential) and a negative curvature (second derivative of the potential), when the configuration is a saddle point and not a minimum. A minimum would have zero force and a positive curvature.