only positive values of every molecule

Posted In Materials Square
  • Samir Hamad 6 months ago

    I noticed that after every geometry optimization the enthalpy, entropy and free energy is always positive. is this a bug? for example:https://www.materialssquare.com/work/29076

    acetone has a positive enthalpy in this optimization but in reality it has a negative value. 

  • Samir Hamad 6 months ago

    and by the way. the entropy of acetone in this example seems really really high. is electronvolt the true unit of the compounds?

  • Materials Square 6 months ago

    Hello. Thank you for using Materials Square.

     

    But there is no problem with the example and the simulation modules.

    The free energy in optimization is negative. Could you let me know what you indicated about the positive free energy? Please check whether you confused the BDE calculation and optimization.

    The module brings the Entropy and enthalpy data from the GAMESS output file. I recommend that you perform your calculation and manually find the value. The following link will be helpful.

    https://medium.com/swlh/introduction-to-molecular-modelling-part-6-calculating-free-energy-of-activation-c862c2db7235

     

    The energy value appears as a positive in the BDE calculation because the product is stable. In fact, if you find the reference, most bonds are expressed as positive values. If BDE is negative, the bond will dissociate really quickly, even formed.

    As you know, the BDE of the C=O bond is 732 kJ/mol. Converting this to eV, it is about 7.59 eV. If so, the example calculation result of 7.646 eV is acceptable.

     

    If you feel the eV is awkward, please click the unit converter icon to the far right of the module selector at the bottom of the work. You can easily convert units.

  • Samir Hamad 6 months ago

    Hello,

    I made a screenshot to demonstrate my problem.

    https://prnt.sc/26j5vxm

    The symbol H stands for enthalpy of formation or did i interpret the symbol wrong?

    The enthalpy of formation H and free energy of formation G displays a positive value and i am confused about that because the experimental value is about -200 kj/mol for the enthalpy of formation

     

  • Materials Square 6 months ago

    Hello.

    The values are the correction term.

    Have you read the link I provided? Specifically, you can use the G correction term to obtain the Gibbs free energy.

    You should add the G correction term to the total energy. Then you can find the calculated Gibbs free energy is negative.

    We'll change the expression to make users do not confuse.

  • Samir Hamad 6 months ago

    Hello dear materials square

     

    I did only read 90% of the article you provided for me and missed the 10% that was actually solving my problem. I am sorry for the inconvenience i caused and thank you for the patience and great service. In a few days my creditcard will arrive so i can continue with my calculations:)

  • Materials Square 6 months ago

    Thank you. I'm glad to hear you have solved your problem!

    Please leave a new article if you have further questions.

    Thank you again for using Materials Square.

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