How to Resolve Convergence Failures in Gaussian NH3+ Calculations?

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SUMMARY

The discussion focuses on resolving convergence failures in Gaussian calculations for the NH3+ cation. The user employed the route section "#MP2 cc-PVDZ SCF=qc" to mitigate strict convergence criteria but still encountered a failure after approximately 2 hours and 44 minutes. The output indicated an error related to curvature, suggesting issues with the potential energy surface. The user seeks clarification on the implications of curvature and its relation to the planar nature of NH3+.

PREREQUISITES
  • Familiarity with Gaussian software for quantum chemistry calculations.
  • Understanding of SCF (Self-Consistent Field) methods and their parameters.
  • Knowledge of MP2 (Møller-Plesset perturbation theory) and its application in electronic structure calculations.
  • Basic concepts of potential energy surfaces and molecular geometry.
NEXT STEPS
  • Research the implications of curvature in potential energy surfaces in quantum chemistry.
  • Explore alternative SCF methods and their effectiveness in Gaussian, such as SCF=Fast or SCF=NoVar.
  • Investigate the impact of molecular geometry on convergence in Gaussian calculations.
  • Learn about the use of different basis sets, such as cc-PVDZ, and their influence on computational results.
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Chemists, computational scientists, and researchers working with quantum chemistry calculations, particularly those using Gaussian software to study molecular systems like NH3+.

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Homework Statement


I am using gaussian to calculate the potential energy surface for NH3+, but I have been having troubles with convergence failures. I did some searching on the internet and found that using SCF=qc can sometimes help with convergence failures since it instruct the program to have less strict criteria. So here's what I entered:

Route Section: #MP2 cc-PVDZ SCF=qc Scan Pop=Reg Guess=INDO

Title: Ammonia Cation Scan

Charge, Multiplicity: 1 2

N
X 1 1.0
H 1 nh 2 xnh
H 1 nh 2 xnh 3 120.0
H 1 nh 2 xnh 3 -120.0

nh 0.5 60 .04
xnh 29.0 60 1.0

And the scan failed after about 2 hrs. and 44 min. I looked at the output file, and it said that the error was due to the curvature, and I'm not quite sure what that means. Any suggestions?


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The Attempt at a Solution

 
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Anyone? Is this maybe due to the fact that NH3+ is planar?
 

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