abstrakt w j. angielskim: |
The linear [
χ
(1)
] and second-order nonlinear [
χ
(2)
]
optical susceptibilities of the 2-methyl-4-nitroaniline (MNA)
crystal are calculated within the local
fi
eld theory, which consists
of
fi
rst computing the molecular properties, accounting for the
dressing e
ff
ects of the surroundings, and then taking into account
the local
fi
eld e
ff
ects. Several aspects of these calculations are
tackled with the aim of monitoring the convergence of the
χ
(1)
and
χ
(2)
predictions with respect to experiment by accounting for
the e
ff
ects of (i) the dressing
fi
eld within successive
approximations, of (ii) the
fi
rst-order ZPVA corrections, and of
(iii) the geometry. With respect to the reference CCSD-based
results, besides double hybrid functionals, the most reliable
exchange-correlation functionals are LC-BLYP for the static
χ
(1)
and CAM-B3LYP (and M05-2X, to a lesser extent) for the
dynamic
χ
(1)
but they strongly underestimate
χ
(2)
. Double hybrids perform better for
χ
(2)
but not necessarily for
χ
(1)
, and,
moreover, their performances are much similar to MP2, which is known to slightly overestimate
β
, with respect to high-level
coupled-clusters calculations and, therefore,
χ
(2)
. Other XC functionals with less HF exchange perform poorly with
overestimations/underestimations of
χ
(1)
/
χ
(2)
, whereas the HF method leads to underestimations of both. The
fi
rst-order ZPVA
corrections, estimated at the B3LYP level, are usually small but not negligible. Indeed, after ZPVA corrections, the molecular
polarizabilities and
fi
rst hyperpolarizabilities increase by 2% and 5%, respectively, whereas their impact is magni
fi
ed on the
macroscopic responses with enhancements of
χ
(1)
by up to 5% and of
χ
(2)
by as much as 10%
−
12% at
λ
= 1064 nm. The
geometry plays also a key role in view of predicting accurate susceptibilities, particularly for push
−
pull
π
-conjugated compounds
such as MNA. So, the geometry optimized using periodic boundary conditions is characterized by an overestimated bond length
alternation, which gives larger molecular properties and even larger macroscopic responses, because of the local
fi
eld factor
ampli
fi
cation e
ff
ects. Our best estimates based on experimental geometries, charge dressing
fi
eld, ZPVA correction, and CCSD
molecular properties lead to an overestimation of
χ
(1)
by 12% in the static limit and 7% at
λ
= 1064 nm. For
χ
(2)
, the di
ff
erence,
with respect to the experiment, is satisfactory and of the order of one standard deviation. |