There are several possible reasons why you may have obtained a significantly larger hyperpolarizability value for urea compared to the values reported in the literature:

1. Basis set: The choice of basis set can have a significant effect on the calculated hyperpolarizability value. Make sure that you have used a basis set that is appropriate for the size and complexity of the molecule. You may want to try using a larger basis set or a different basis set altogether to see if that affects your results.

2. Method: The choice of computational method can also affect the calculated hyperpolarizability value. Make sure that you have used a method that is appropriate for the level of theory that you are interested in. You may want to try using a different method, such as a different density functional theory (DFT) method or a different post-Hartree-Fock method, to see if that affects your results.

3. Convergence: Ensure that your calculations have converged by checking that the total energy, dipole moment, and other relevant properties have converged with respect to basis set and level of theory. You may need to increase the size of your basis set, increase the level of theory, or adjust other computational parameters to achieve convergence.

4. Experimental conditions: The reported hyperpolarizability values in the literature may have been measured under different experimental conditions than those used in your calculation. Make sure that you have accounted for any differences in experimental conditions, such as the wavelength of the incident radiation.

5. Errors in implementation: It is possible that there may be errors in the implementation of the polar=dcshg keyword or in the Gaussian software itself. You may want to consult the Gaussian documentation, check your input file for errors, or contact the Gaussian support team for assistance.

It is important to note that hyperpolarizability values are highly sensitive to computational parameters and can vary significantly between different methods and basis sets. Therefore, it is recommended to perform a thorough validation of the computational method and basis set before making any conclusions about the accuracy of the calculated hyperpolarizability value.

@lamia fatima thank you so much

@Lamia,

Well articulated